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perlglossary - Perl Glossary


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Perl Programmers Reference Guide                  PERLGLOSSARY(1)

     perlglossary - Perl Glossary

     A glossary of terms (technical and otherwise) used in the
     Perl documentation.  Other useful sources include the Free
     On-Line Dictionary of Computing
     <>, the Jargon
     File <>, and Wikipedia

     accessor methods
         A "method" used to indirectly inspect or update an
         "object"'s state (its instance variables).

     actual arguments
         The scalar values that you supply to a "function" or
         "subroutine" when you call it.  For instance, when you
         call "power("puff")", the string "puff" is the actual
         argument.  See also "argument" and "formal arguments".

     address operator
         Some languages work directly with the memory addresses
         of values, but this can be like playing with fire.  Perl
         provides a set of asbestos gloves for handling all
         memory management.  The closest to an address operator
         in Perl is the backslash operator, but it gives you a
         "hard reference", which is much safer than a memory

         A well-defined sequence of steps, clearly enough
         explained that even a computer could do them.

         A nickname for something, which behaves in all ways as
         though you'd used the original name instead of the
         nickname.  Temporary aliases are implicitly created in
         the loop variable for "foreach" loops, in the $_
         variable for map or grep operators, in $a and $b during
         sort's comparison function, and in each element of @_
         for the "actual arguments" of a subroutine call.
         Permanent aliases are explicitly created in packages by
         importing symbols or by assignment to typeglobs.
         Lexically scoped aliases for package variables are
         explicitly created by the our declaration.

         A list of possible choices from which you may select
         only one, as in "Would you like door A, B, or C?"
         Alternatives in regular expressions are separated with a

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         single vertical bar: "|".  Alternatives in normal Perl
         expressions are separated with a double vertical bar:
         "||".  Logical alternatives in "Boolean" expressions are
         separated with either "||" or "or".

         Used to describe a "referent" that is not directly
         accessible through a named "variable".  Such a referent
         must be indirectly accessible through at least one "hard
         reference".  When the last hard reference goes away, the
         anonymous referent is destroyed without pity.

         The kind of computer you're working on, where one "kind"
         of computer means all those computers sharing a
         compatible machine language.  Since Perl programs are
         (typically) simple text files, not executable images, a
         Perl program is much less sensitive to the architecture
         it's running on than programs in other languages, such
         as C, that are compiled into machine code.  See also
         "platform" and "operating system".

         A piece of data supplied to a program, "subroutine",
         "function", or "method" to tell it what it's supposed to
         do.  Also called a "parameter".

         The name of the array containing the "argument" "vector"
         from the command line.  If you use the empty "<>"
         operator, "ARGV" is the name of both the "filehandle"
         used to traverse the arguments and the "scalar"
         containing the name of the current input file.

     arithmetical operator
         A "symbol" such as "+" or "/" that tells Perl to do the
         arithmetic you were supposed to learn in grade school.

         An ordered sequence of values, stored such that you can
         easily access any of the values using an integer
         "subscript" that specifies the value's "offset" in the

     array context
         An archaic expression for what is more correctly
         referred to as "list context".

         The American Standard Code for Information Interchange
         (a 7-bit character set adequate only for poorly
         representing English text).  Often used loosely to

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         describe the lowest 128 values of the various ISO-8859-X
         character sets, a bunch of mutually incompatible 8-bit
         codes sometimes described as half ASCII.  See also

         A component of a "regular expression" that must be true
         for the pattern to match but does not necessarily match
         any characters itself.  Often used specifically to mean
         a "zero width" assertion.

         An "operator" whose assigned mission in life is to
         change the value of a "variable".

     assignment operator
         Either a regular "assignment", or a compound "operator"
         composed of an ordinary assignment and some other
         operator, that changes the value of a variable in place,
         that is, relative to its old value.  For example, "$a +=
         2" adds 2 to $a.

     associative array
         See "hash".  Please.

         Determines whether you do the left "operator" first or
         the right "operator" first when you have "A "operator" B
         "operator" C" and the two operators are of the same
         precedence.  Operators like "+" are left associative,
         while operators like "**" are right associative.  See
         perlop for a list of operators and their associativity.

         Said of events or activities whose relative temporal
         ordering is indeterminate because too many things are
         going on at once.  Hence, an asynchronous event is one
         you didn't know when to expect.

         A "regular expression" component potentially matching a
         "substring" containing one or more characters and
         treated as an indivisible syntactic unit by any
         following "quantifier".  (Contrast with an "assertion"
         that matches something of "zero width" and may not be

     atomic operation
         When Democritus gave the word "atom" to the indivisible
         bits of matter, he meant literally something that could
         not be cut: a- (not) + tomos (cuttable).  An atomic
         operation is an action that can't be interrupted, not

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         one forbidden in a nuclear-free zone.

         A new feature that allows the declaration of variables
         and subroutines with modifiers as in "sub foo : locked
         method".  Also, another name for an "instance variable"
         of an "object".

         A feature of "operator overloading" of objects, whereby
         the behavior of certain operators can be reasonably
         deduced using more fundamental operators.  This assumes
         that the overloaded operators will often have the same
         relationships as the regular operators.  See perlop.

         To add one to something automatically, hence the name of
         the "++" operator.  To instead subtract one from
         something automatically is known as an "autodecrement".

         To load on demand.  (Also called "lazy" loading.)
         Specifically, to call an AUTOLOAD subroutine on behalf
         of an undefined subroutine.

         To split a string automatically, as the -a "switch" does
         when running under -p or -n in order to emulate "awk".
         (See also the AutoSplit module, which has nothing to do
         with the -a switch, but a lot to do with autoloading.)

         A Greco-Roman word meaning "to bring oneself to life".
         In Perl, storage locations (lvalues) spontaneously
         generate themselves as needed, including the creation of
         any "hard reference" values to point to the next level
         of storage.  The assignment "$a[5][5][5][5][5] =
         "quintet"" potentially creates five scalar storage
         locations, plus four references (in the first four
         scalar locations) pointing to four new anonymous arrays
         (to hold the last four scalar locations).  But the point
         of autovivification is that you don't have to worry
         about it.

     AV  Short for "array value", which refers to one of Perl's
         internal data types that holds an "array".  The "AV"
         type is a subclass of "SV".

     awk Descriptive editing term--short for "awkward".  Also
         coincidentally refers to a venerable text-processing
         language from which Perl derived some of its high-level

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         A substring captured by a subpattern within unadorned
         parentheses in a "regex".  Backslashed decimal numbers
         ("\1", "\2", etc.)  later in the same pattern refer back
         to the corresponding subpattern in the current match.
         Outside the pattern, the numbered variables ($1, $2,
         etc.) continue to refer to these same values, as long as
         the pattern was the last successful match of the current
         dynamic scope.

         The practice of saying, "If I had to do it all over, I'd
         do it differently," and then actually going back and
         doing it all over differently.  Mathematically speaking,
         it's returning from an unsuccessful recursion on a tree
         of possibilities.  Perl backtracks when it attempts to
         match patterns with a "regular expression", and its
         earlier attempts don't pan out.  See "Backtracking" in

     backward compatibility
         Means you can still run your old program because we
         didn't break any of the features or bugs it was relying

         A word sufficiently ambiguous to be deemed illegal under
         use strict 'subs'.  In the absence of that stricture, a
         bareword is treated as if quotes were around it.

     base class
         A generic "object" type; that is, a "class" from which
         other, more specific classes are derived genetically by
         "inheritance".  Also called a "superclass" by people who
         respect their ancestors.

         From Swift: someone who eats eggs big end first.  Also
         used of computers that store the most significant "byte"
         of a word at a lower byte address than the least
         significant byte.  Often considered superior to little-
         endian machines.  See also "little-endian".

         Having to do with numbers represented in base 2.  That
         means there's basically two numbers, 0 and 1.  Also used
         to describe a "non-text file", presumably because such a
         file makes full use of all the binary bits in its bytes.
         With the advent of "Unicode", this distinction, already
         suspect, loses even more of its meaning.

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     binary operator
         An "operator" that takes two operands.

         To assign a specific "network address" to a "socket".

     bit An integer in the range from 0 to 1, inclusive.  The
         smallest possible unit of information storage.  An
         eighth of a "byte" or of a dollar.  (The term "Pieces of
         Eight" comes from being able to split the old Spanish
         dollar into 8 bits, each of which still counted for
         money.  That's why a 25-cent piece today is still "two

     bit shift
         The movement of bits left or right in a computer word,
         which has the effect of multiplying or dividing by a
         power of 2.

     bit string
         A sequence of bits that is actually being thought of as
         a sequence of bits, for once.

         In corporate life, to grant official approval to a
         thing, as in, "The VP of Engineering has blessed our
         WebCruncher project." Similarly in Perl, to grant
         official approval to a "referent" so that it can
         function as an "object", such as a WebCruncher object.
         See "bless" in perlfunc.

         What a "process" does when it has to wait for something:
         "My process blocked waiting for the disk."  As an
         unrelated noun, it refers to a large chunk of data, of a
         size that the "operating system" likes to deal with
         (normally a power of two such as 512 or 8192).
         Typically refers to a chunk of data that's coming from
         or going to a disk file.

         A syntactic construct consisting of a sequence of Perl
         statements that is delimited by braces.  The "if" and
         "while" statements are defined in terms of BLOCKs, for
         instance.  Sometimes we also say "block" to mean a
         lexical scope; that is, a sequence of statements that
         act like a "BLOCK", such as within an eval or a file,
         even though the statements aren't delimited by braces.

     block buffering
         A method of making input and output efficient by passing
         one "block" at a time.  By default, Perl does block

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         buffering to disk files.  See "buffer" and "command

         A value that is either "true" or "false".

     Boolean context
         A special kind of "scalar context" used in conditionals
         to decide whether the "scalar value" returned by an
         expression is "true" or "false".  Does not evaluate as
         either a string or a number.  See "context".

         A spot in your program where you've told the debugger to
         stop execution so you can poke around and see whether
         anything is wrong yet.

         To send a "datagram" to multiple destinations

     BSD A psychoactive drug, popular in the 80s, probably
         developed at U. C. Berkeley or thereabouts.  Similar in
         many ways to the prescription-only medication called
         "System V", but infinitely more useful.  (Or, at least,
         more fun.)  The full chemical name is "Berkeley Standard

         A location in a "hash table" containing (potentially)
         multiple entries whose keys "hash" to the same hash
         value according to its hash function.  (As internal
         policy, you don't have to worry about it, unless you're
         into internals, or policy.)

         A temporary holding location for data.  Block buffering
         means that the data is passed on to its destination
         whenever the buffer is full.  Line buffering means that
         it's passed on whenever a complete line is received.
         Command buffering means that it's passed every time you
         do a print command (or equivalent).  If your output is
         unbuffered, the system processes it one byte at a time
         without the use of a holding area.  This can be rather

         A "function" that is predefined in the language.  Even
         when hidden by "overriding", you can always get at a
         built-in function by qualifying its name with the
         "CORE::" pseudo-package.

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         A group of related modules on "CPAN".  (Also, sometimes
         refers to a group of command-line switches grouped into
         one "switch cluster".)

         A piece of data worth eight bits in most places.

         A pidgin-like language spoken among 'droids when they
         don't wish to reveal their orientation (see "endian").
         Named after some similar languages spoken (for similar
         reasons) between compilers and interpreters in the late
         20th century.  These languages are characterized by
         representing everything as a non-architecture-dependent
         sequence of bytes.

     C   A language beloved by many for its inside-out "type"
         definitions, inscrutable "precedence" rules, and heavy
         "overloading" of the function-call mechanism.  (Well,
         actually, people first switched to C because they found
         lowercase identifiers easier to read than upper.)  Perl
         is written in C, so it's not surprising that Perl
         borrowed a few ideas from it.

     C preprocessor
         The typical C compiler's first pass, which processes
         lines beginning with "#" for conditional compilation and
         macro definition and does various manipulations of the
         program text based on the current definitions.  Also
         known as cpp(1).

     call by reference
         An "argument"-passing mechanism in which the "formal
         arguments" refer directly to the "actual arguments", and
         the "subroutine" can change the actual arguments by
         changing the formal arguments.  That is, the formal
         argument is an "alias" for the actual argument.  See
         also "call by value".

     call by value
         An "argument"-passing mechanism in which the "formal
         arguments" refer to a copy of the "actual arguments",
         and the "subroutine" cannot change the actual arguments
         by changing the formal arguments.  See also "call by

         A "handler" that you register with some other part of
         your program in the hope that the other part of your
         program will "trigger" your handler when some event of

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         interest transpires.

         Reduced to a standard form to facilitate comparison.

         The use of parentheses around a "subpattern" in a
         "regular expression" to store the matched "substring" as
         a "backreference".  (Captured strings are also returned
         as a list in "list context".)

         A small integer representative of a unit of orthography.
         Historically, characters were usually stored as fixed-
         width integers (typically in a byte, or maybe two,
         depending on the character set), but with the advent of
         UTF-8, characters are often stored in a variable number
         of bytes depending on the size of the integer that
         represents the character.  Perl manages this
         transparently for you, for the most part.

     character class
         A square-bracketed list of characters used in a "regular
         expression" to indicate that any character of the set
         may occur at a given point.  Loosely, any predefined set
         of characters so used.

     character property
         A predefined "character class" matchable by the "\p"
         "metasymbol".  Many standard properties are defined for

     circumfix operator
         An "operator" that surrounds its "operand", like the
         angle operator, or parentheses, or a hug.

         A user-defined "type", implemented in Perl via a
         "package" that provides (either directly or by
         inheritance) methods (that is, subroutines) to handle
         instances of the class (its objects).  See also

     class method
         A "method" whose "invocant" is a "package" name, not an
         "object" reference.  A method associated with the class
         as a whole.

         In networking, a "process" that initiates contact with a
         "server" process in order to exchange data and perhaps
         receive a service.

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         A "cluster" used to restrict the scope of a "regular
         expression modifier".

         An "anonymous" subroutine that, when a reference to it
         is generated at run time, keeps track of the identities
         of externally visible lexical variables even after those
         lexical variables have supposedly gone out of "scope".
         They're called "closures" because this sort of behavior
         gives mathematicians a sense of closure.

         A parenthesized "subpattern" used to group parts of a
         "regular expression" into a single "atom".

         The word returned by the ref function when you apply it
         to a reference to a subroutine.  See also "CV".

     code generator
         A system that writes code for you in a low-level
         language, such as code to implement the backend of a
         compiler.  See "program generator".

     code subpattern
         A "regular expression" subpattern whose real purpose is
         to execute some Perl code, for example, the "(?{...})"
         and "(??{...})" subpatterns.

     collating sequence
         The order into which characters sort.  This is used by
         "string" comparison routines to decide, for example,
         where in this glossary to put "collating sequence".

         In "shell" programming, the syntactic combination of a
         program name and its arguments.  More loosely, anything
         you type to a shell (a command interpreter) that starts
         it doing something.  Even more loosely, a Perl
         "statement", which might start with a "label" and
         typically ends with a semicolon.

     command buffering
         A mechanism in Perl that lets you store up the output of
         each Perl "command" and then flush it out as a single
         request to the "operating system".  It's enabled by
         setting the $| ($AUTOFLUSH) variable to a true value.
         It's used when you don't want data sitting around not
         going where it's supposed to, which may happen because
         the default on a "file" or "pipe" is to use "block

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     command name
         The name of the program currently executing, as typed on
         the command line.  In C, the "command" name is passed to
         the program as the first command-line argument.  In
         Perl, it comes in separately as $0.

     command-line arguments
         The values you supply along with a program name when you
         tell a "shell" to execute a "command".  These values are
         passed to a Perl program through @ARGV.

         A remark that doesn't affect the meaning of the program.
         In Perl, a comment is introduced by a "#" character and
         continues to the end of the line.

     compilation unit
         The "file" (or "string", in the case of eval) that is
         currently being compiled.

     compile phase
         Any time before Perl starts running your main program.
         See also "run phase".  Compile phase is mostly spent in
         "compile time", but may also be spent in "run time" when
         "BEGIN" blocks, use declarations, or constant
         subexpressions are being evaluated.  The startup and
         import code of any use declaration is also run during
         compile phase.

     compile time
         The time when Perl is trying to make sense of your code,
         as opposed to when it thinks it knows what your code
         means and is merely trying to do what it thinks your
         code says to do, which is "run time".

         Strictly speaking, a program that munches up another
         program and spits out yet another file containing the
         program in a "more executable" form, typically
         containing native machine instructions.  The perl
         program is not a compiler by this definition, but it
         does contain a kind of compiler that takes a program and
         turns it into a more executable form (syntax trees)
         within the perl process itself, which the "interpreter"
         then interprets.  There are, however, extension modules
         to get Perl to act more like a "real" compiler.  See O.

         A "constructor" for a "referent" that isn't really an
         "object", like an anonymous array or a hash (or a
         sonata, for that matter).  For example, a pair of braces
         acts as a composer for a hash, and a pair of brackets

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         acts as a composer for an array.  See "Making
         References" in perlref.

         The process of gluing one cat's nose to another cat's
         tail.  Also, a similar operation on two strings.

         Something "iffy".  See "Boolean context".

         In telephony, the temporary electrical circuit between
         the caller's and the callee's phone.  In networking, the
         same kind of temporary circuit between a "client" and a

         As a noun, a piece of syntax made up of smaller pieces.
         As a transitive verb, to create an "object" using a

         Any "class method", instance "method", or "subroutine"
         that composes, initializes, blesses, and returns an
         "object".  Sometimes we use the term loosely to mean a

         The surroundings, or environment.  The context given by
         the surrounding code determines what kind of data a
         particular "expression" is expected to return.  The
         three primary contexts are "list context", "scalar
         context", and "void context".  Scalar context is
         sometimes subdivided into "Boolean context", "numeric
         context", "string context", and "void context".  There's
         also a "don't care" scalar context (which is dealt with
         in Programming Perl, Third Edition, Chapter 2, "Bits and
         Pieces" if you care).

         The treatment of more than one physical "line" as a
         single logical line.  "Makefile" lines are continued by
         putting a backslash before the "newline".  Mail headers
         as defined by RFC 822 are continued by putting a space
         or tab after the newline.  In general, lines in Perl do
         not need any form of continuation mark, because
         "whitespace" (including newlines) is gleefully ignored.

     core dump
         The corpse of a "process", in the form of a file left in
         the "working directory" of the process, usually as a

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         result of certain kinds of fatal error.

         The Comprehensive Perl Archive Network.  (See "What
         modules and extensions are available for Perl?  What is
         CPAN?  What does CPAN/src/... mean?" in perlfaq2).

         Someone who breaks security on computer systems.  A
         cracker may be a true "hacker" or only a "script

     current package
         The "package" in which the current statement is
         compiled.  Scan backwards in the text of your program
         through the current lexical scope or any enclosing
         lexical scopes till you find a package declaration.
         That's your current package name.

     current working directory
         See "working directory".

     currently selected output channel
         The last "filehandle" that was designated with
         select("FILEHANDLE"); "STDOUT", if no filehandle has
         been selected.

     CV  An internal "code value" typedef, holding a
         "subroutine".  The "CV" type is a subclass of "SV".

     dangling statement
         A bare, single "statement", without any braces, hanging
         off an "if" or "while" conditional.  C allows them.
         Perl doesn't.

     data structure
         How your various pieces of data relate to each other and
         what shape they make when you put them all together, as
         in a rectangular table or a triangular-shaped tree.

     data type
         A set of possible values, together with all the
         operations that know how to deal with those values.  For
         example, a numeric data type has a certain set of
         numbers that you can work with and various mathematical
         operations that you can do on the numbers but would make
         little sense on, say, a string such as "Kilroy".
         Strings have their own operations, such as
         "concatenation".  Compound types made of a number of
         smaller pieces generally have operations to compose and
         decompose them, and perhaps to rearrange them.  Objects

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         that model things in the real world often have
         operations that correspond to real activities.  For
         instance, if you model an elevator, your elevator object
         might have an "open_door()" "method".

         A packet of data, such as a "UDP" message, that (from
         the viewpoint of the programs involved) can be sent
         independently over the network.  (In fact, all packets
         are sent independently at the "IP" level, but "stream"
         protocols such as "TCP" hide this from your program.)

     DBM Stands for "Data Base Management" routines, a set of
         routines that emulate an "associative array" using disk
         files.  The routines use a dynamic hashing scheme to
         locate any entry with only two disk accesses.  DBM files
         allow a Perl program to keep a persistent "hash" across
         multiple invocations.  You can tie your hash variables
         to various DBM implementations--see AnyDBM_File and

         An "assertion" that states something exists and perhaps
         describes what it's like, without giving any commitment
         as to how or where you'll use it.  A declaration is like
         the part of your recipe that says, "two cups flour, one
         large egg, four or five tadpoles..."  See "statement"
         for its opposite.  Note that some declarations also
         function as statements.  Subroutine declarations also
         act as definitions if a body is supplied.

         To subtract a value from a variable, as in "decrement
         $x" (meaning to remove 1 from its value) or "decrement
         $x by 3".

         A "value" chosen for you if you don't supply a value of
         your own.

         Having a meaning.  Perl thinks that some of the things
         people try to do are devoid of meaning, in particular,
         making use of variables that have never been given a
         "value" and performing certain operations on data that
         isn't there.  For example, if you try to read data past
         the end of a file, Perl will hand you back an undefined
         value.  See also "false" and "defined" in perlfunc.

         A "character" or "string" that sets bounds to an
         arbitrarily-sized textual object, not to be confused

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         with a "separator" or "terminator".  "To delimit" really
         just means "to surround" or "to enclose" (like these
         parentheses are doing).

     deprecated modules and features
         Deprecated modules and features are those which were
         part of a stable release, but later found to be subtly
         flawed, and which should be avoided.  They are subject
         to removal and/or bug-incompatible reimplementation in
         the next major release (but they will be preserved
         through maintenance releases).  Deprecation warnings are
         issued under -w or "use diagnostics", and notices are
         found in perldeltas, as well as various other PODs.
         Coding practices that misuse features, such as "my $foo
         if 0", can also be deprecated.

         A fancy computer science term meaning "to follow a
         "reference" to what it points to".  The "de" part of it
         refers to the fact that you're taking away one level of

     derived class
         A "class" that defines some of its methods in terms of a
         more generic class, called a "base class".  Note that
         classes aren't classified exclusively into base classes
         or derived classes: a class can function as both a
         derived class and a base class simultaneously, which is
         kind of classy.

         See "file descriptor".

         To deallocate the memory of a "referent" (first
         triggering its "DESTROY" method, if it has one).

         A special "method" that is called when an "object" is
         thinking about destroying itself.  A Perl program's
         "DESTROY" method doesn't do the actual destruction; Perl
         just triggers the method in case the "class" wants to do
         any associated cleanup.

         A whiz-bang hardware gizmo (like a disk or tape drive or
         a modem or a joystick or a mouse) attached to your
         computer, that the "operating system" tries to make look
         like a "file" (or a bunch of files).  Under Unix, these
         fake files tend to live in the /dev directory.


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         A "pod" directive.  See perlpod.

         A special file that contains other files.  Some
         operating systems call these "folders", "drawers", or

     directory handle
         A name that represents a particular instance of opening
         a directory to read it, until you close it.  See the
         opendir function.

         To send something to its correct destination.  Often
         used metaphorically to indicate a transfer of
         programmatic control to a destination selected
         algorithmically, often by lookup in a table of function
         references or, in the case of object methods, by
         traversing the inheritance tree looking for the most
         specific definition for the method.

         A standard, bundled release of a system of software.
         The default usage implies source code is included.  If
         that is not the case, it will be called a "binary-only"

     (to be) dropped modules
         When Perl 5 was first released (see perlhistory),
         several modules were included, which have now fallen out
         of common use.  It has been suggested that these modules
         should be removed, since the distribution became rather
         large, and the common criterion for new module additions
         is now limited to modules that help to build, test, and
         extend perl itself.  Furthermore, the CPAN (which didn't
         exist at the time of Perl 5.0) can become the new home
         of dropped modules. Dropping modules is currently not an
         option, but further developments may clear the last

         An enchantment, illusion, phantasm, or jugglery.  Said
         when Perl's magical "dwimmer" effects don't do what you
         expect, but rather seem to be the product of arcane
         dweomercraft, sorcery, or wonder working.  [From Old

         DWIM is an acronym for "Do What I Mean", the principle
         that something should just do what you want it to do
         without an undue amount of fuss.  A bit of code that
         does "dwimming" is a "dwimmer".  Dwimming can require a

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         great deal of behind-the-scenes magic, which (if it
         doesn't stay properly behind the scenes) is called a
         "dweomer" instead.

     dynamic scoping
         Dynamic scoping works over a dynamic scope, making
         variables visible throughout the rest of the "block" in
         which they are first used and in any subroutines that
         are called by the rest of the block.  Dynamically scoped
         variables can have their values temporarily changed (and
         implicitly restored later) by a local operator.
         (Compare "lexical scoping".)  Used more loosely to mean
         how a subroutine that is in the middle of calling
         another subroutine "contains" that subroutine at "run

         Derived from many sources.  Some would say too many.

         A basic building block.  When you're talking about an
         "array", it's one of the items that make up the array.

         When something is contained in something else,
         particularly when that might be considered surprising:
         "I've embedded a complete Perl interpreter in my

     empty subclass test
         The notion that an empty "derived class" should behave
         exactly like its "base class".

     en passant
         When you change a "value" as it is being copied.  [From
         French, "in passing", as in the exotic pawn-capturing
         maneuver in chess.]

         The veil of abstraction separating the "interface" from
         the "implementation" (whether enforced or not), which
         mandates that all access to an "object"'s state be
         through methods alone.

         See "little-endian" and "big-endian".

         The collective set of environment variables your
         "process" inherits from its parent.  Accessed via %ENV.

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     environment variable
         A mechanism by which some high-level agent such as a
         user can pass its preferences down to its future
         offspring (child processes, grandchild processes, great-
         grandchild processes, and so on).  Each environment
         variable is a "key"/"value" pair, like one entry in a

     EOF End of File.  Sometimes used metaphorically as the
         terminating string of a "here document".

         The error number returned by a "syscall" when it fails.
         Perl refers to the error by the name $! (or $OS_ERROR if
         you use the English module).

         See "exception" or "fatal error".

     escape sequence
         See "metasymbol".

         A fancy term for an error.  See "fatal error".

     exception handling
         The way a program responds to an error.  The exception
         handling mechanism in Perl is the eval operator.

         To throw away the current "process"'s program and
         replace it with another without exiting the process or
         relinquishing any resources held (apart from the old
         memory image).

     executable file
         A "file" that is specially marked to tell the "operating
         system" that it's okay to run this file as a program.
         Usually shortened to "executable".

         To run a program or "subroutine".  (Has nothing to do
         with the kill built-in, unless you're trying to run a
         "signal handler".)

     execute bit
         The special mark that tells the operating system it can
         run this program.  There are actually three execute bits
         under Unix, and which bit gets used depends on whether
         you own the file singularly, collectively, or not at

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     exit status
         See "status".

         To make symbols from a "module" available for "import"
         by other modules.

         Anything you can legally say in a spot where a "value"
         is required.  Typically composed of literals, variables,
         operators, functions, and "subroutine" calls, not
         necessarily in that order.

         A Perl module that also pulls in compiled C or C++ code.
         More generally, any experimental option that can be
         compiled into Perl, such as multithreading.

         In Perl, any value that would look like "" or "0" if
         evaluated in a string context.  Since undefined values
         evaluate to "", all undefined values are false, but not
         all false values are undefined.

     FAQ Frequently Asked Question (although not necessarily
         frequently answered, especially if the answer appears in
         the Perl FAQ shipped standard with Perl).

     fatal error
         An uncaught "exception", which causes termination of the
         "process" after printing a message on your "standard
         error" stream.  Errors that happen inside an eval are
         not fatal.  Instead, the eval terminates after placing
         the exception message in the $@ ($EVAL_ERROR) variable.
         You can try to provoke a fatal error with the die
         operator (known as throwing or raising an exception),
         but this may be caught by a dynamically enclosing eval.
         If not caught, the die becomes a fatal error.

         A single piece of numeric or string data that is part of
         a longer "string", "record", or "line".  Variable-width
         fields are usually split up by separators (so use split
         to extract the fields), while fixed-width fields are
         usually at fixed positions (so use unpack).  Instance
         variables are also known as fields.

         First In, First Out.  See also "LIFO".  Also, a nickname
         for a "named pipe".

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         A named collection of data, usually stored on disk in a
         "directory" in a "filesystem".  Roughly like a document,
         if you're into office metaphors.  In modern filesystems,
         you can actually give a file more than one name.  Some
         files have special properties, like directories and

     file descriptor
         The little number the "operating system" uses to keep
         track of which opened "file" you're talking about.  Perl
         hides the file descriptor inside a "standard I/O" stream
         and then attaches the stream to a "filehandle".

     file test operator
         A built-in unary operator that you use to determine
         whether something is "true" about a file, such as "-o
         $filename" to test whether you're the owner of the file.

         A "wildcard" match on filenames.  See the glob function.

         An identifier (not necessarily related to the real name
         of a file) that represents a particular instance of
         opening a file until you close it.  If you're going to
         open and close several different files in succession,
         it's fine to open each of them with the same filehandle,
         so you don't have to write out separate code to process
         each file.

         One name for a file.  This name is listed in a
         "directory", and you can use it in an open to tell the
         "operating system" exactly which file you want to open,
         and associate the file with a "filehandle" which will
         carry the subsequent identity of that file in your
         program, until you close it.

         A set of directories and files residing on a partition
         of the disk.  Sometimes known as a "partition".  You can
         change the file's name or even move a file around from
         directory to directory within a filesystem without
         actually moving the file itself, at least under Unix.

         A program designed to take a "stream" of input and
         transform it into a stream of output.

         We tend to avoid this term because it means so many

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         things.  It may mean a command-line "switch" that takes
         no argument itself (such as Perl's -n and -p flags) or,
         less frequently, a single-bit indicator (such as the
         "O_CREAT" and "O_EXCL" flags used in sysopen).

     floating point
         A method of storing numbers in "scientific notation",
         such that the precision of the number is independent of
         its magnitude (the decimal point "floats").  Perl does
         its numeric work with floating-point numbers (sometimes
         called "floats"), when it can't get away with using
         integers.  Floating-point numbers are mere
         approximations of real numbers.

         The act of emptying a "buffer", often before it's full.

         Far More Than Everything You Ever Wanted To Know.  An
         exhaustive treatise on one narrow topic, something of a
         super-"FAQ".  See Tom for far more.

         To create a child "process" identical to the parent
         process at its moment of conception, at least until it
         gets ideas of its own.  A thread with protected memory.

     formal arguments
         The generic names by which a "subroutine" knows its
         arguments.  In many languages, formal arguments are
         always given individual names, but in Perl, the formal
         arguments are just the elements of an array.  The formal
         arguments to a Perl program are $ARGV[0], $ARGV[1], and
         so on.  Similarly, the formal arguments to a Perl
         subroutine are $_[0], $_[1], and so on.  You may give
         the arguments individual names by assigning the values
         to a my list.  See also "actual arguments".

         A specification of how many spaces and digits and things
         to put somewhere so that whatever you're printing comes
         out nice and pretty.

     freely available
         Means you don't have to pay money to get it, but the
         copyright on it may still belong to someone else (like

     freely redistributable
         Means you're not in legal trouble if you give a bootleg
         copy of it to your friends and we find out about it.  In
         fact, we'd rather you gave a copy to all your friends.

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         Historically, any software that you give away,
         particularly if you make the source code available as
         well.  Now often called "open source software".
         Recently there has been a trend to use the term in
         contradistinction to "open source software", to refer
         only to free software released under the Free Software
         Foundation's GPL (General Public License), but this is
         difficult to justify etymologically.

         Mathematically, a mapping of each of a set of input
         values to a particular output value.  In computers,
         refers to a "subroutine" or "operator" that returns a
         "value".  It may or may not have input values (called

     funny character
         Someone like Larry, or one of his peculiar friends.
         Also refers to the strange prefixes that Perl requires
         as noun markers on its variables.

     garbage collection
         A misnamed feature--it should be called, "expecting your
         mother to pick up after you".  Strictly speaking, Perl
         doesn't do this, but it relies on a reference-counting
         mechanism to keep things tidy.  However, we rarely speak
         strictly and will often refer to the reference-counting
         scheme as a form of garbage collection.  (If it's any
         comfort, when your interpreter exits, a "real" garbage
         collector runs to make sure everything is cleaned up if
         you've been messy with circular references and such.)

     GID Group ID--in Unix, the numeric group ID that the
         "operating system" uses to identify you and members of
         your "group".

         Strictly, the shell's "*" character, which will match a
         "glob" of characters when you're trying to generate a
         list of filenames.  Loosely, the act of using globs and
         similar symbols to do pattern matching.  See also
         "fileglob" and "typeglob".

         Something you can see from anywhere, usually used of
         variables and subroutines that are visible everywhere in
         your program.  In Perl, only certain special variables
         are truly global--most variables (and all subroutines)
         exist only in the current "package".  Global variables
         can be declared with our.  See "our" in perlfunc.

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     global destruction
         The "garbage collection" of globals (and the running of
         any associated object destructors) that takes place when
         a Perl "interpreter" is being shut down.  Global
         destruction should not be confused with the Apocalypse,
         except perhaps when it should.

     glue language
         A language such as Perl that is good at hooking things
         together that weren't intended to be hooked together.

         The size of the pieces you're dealing with, mentally

         A "subpattern" whose "quantifier" wants to match as many
         things as possible.

         Originally from the old Unix editor command for
         "Globally search for a Regular Expression and Print it",
         now used in the general sense of any kind of search,
         especially text searches.  Perl has a built-in grep
         function that searches a list for elements matching any
         given criterion, whereas the grep(1) program searches
         for lines matching a "regular expression" in one or more

         A set of users of which you are a member.  In some
         operating systems (like Unix), you can give certain file
         access permissions to other members of your group.

     GV  An internal "glob value" typedef, holding a "typeglob".
         The "GV" type is a subclass of "SV".

         Someone who is brilliantly persistent in solving
         technical problems, whether these involve golfing,
         fighting orcs, or programming.  Hacker is a neutral
         term, morally speaking.  Good hackers are not to be
         confused with evil crackers or clueless script kiddies.
         If you confuse them, we will presume that you are either
         evil or clueless.

         A "subroutine" or "method" that is called by Perl when
         your program needs to respond to some internal event,
         such as a "signal", or an encounter with an operator
         subject to "operator overloading".  See also "callback".

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     hard reference
         A "scalar" "value" containing the actual address of a
         "referent", such that the referent's "reference" count
         accounts for it.  (Some hard references are held
         internally, such as the implicit reference from one of a
         "typeglob"'s variable slots to its corresponding
         referent.)  A hard reference is different from a
         "symbolic reference".

         An unordered association of "key"/"value" pairs, stored
         such that you can easily use a string "key" to look up
         its associated data "value".  This glossary is like a
         hash, where the word to be defined is the key, and the
         definition is the value.  A hash is also sometimes
         septisyllabically called an "associative array", which
         is a pretty good reason for simply calling it a "hash"

     hash table
         A data structure used internally by Perl for
         implementing associative arrays (hashes) efficiently.
         See also "bucket".

     header file
         A file containing certain required definitions that you
         must include "ahead" of the rest of your program to do
         certain obscure operations.  A C header file has a .h
         extension.  Perl doesn't really have header files,
         though historically Perl has sometimes used translated
         .h files with a .ph extension.  See "require" in
         perlfunc.  (Header files have been superseded by the
         "module" mechanism.)

     here document
         So called because of a similar construct in shells that
         pretends that the lines following the "command" are a
         separate "file" to be fed to the command, up to some
         terminating string.  In Perl, however, it's just a fancy
         form of quoting.

         A number in base 16, "hex" for short.  The digits for 10
         through 16 are customarily represented by the letters
         "a" through "f".  Hexadecimal constants in Perl start
         with "0x".  See also "hex" in perlfunc.

     home directory
         The directory you are put into when you log in.  On a
         Unix system, the name is often placed into $ENV{HOME} or
         $ENV{LOGDIR} by login, but you can also find it with
         "(getpwuid($<))[7]".  (Some platforms do not have a

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         concept of a home directory.)

         The computer on which a program or other data resides.

         Excessive pride, the sort of thing Zeus zaps you for.
         Also the quality that makes you write (and maintain)
         programs that other people won't want to say bad things
         about.  Hence, the third great virtue of a programmer.
         See also "laziness" and "impatience".

     HV  Short for a "hash value" typedef, which holds Perl's
         internal representation of a hash.  The "HV" type is a
         subclass of "SV".

         A legally formed name for most anything in which a
         computer program might be interested.  Many languages
         (including Perl) allow identifiers that start with a
         letter and contain letters and digits.  Perl also counts
         the underscore character as a valid letter.  (Perl also
         has more complicated names, such as "qualified" names.)

         The anger you feel when the computer is being lazy.
         This makes you write programs that don't just react to
         your needs, but actually anticipate them.  Or at least
         that pretend to.  Hence, the second great virtue of a
         programmer.  See also "laziness" and "hubris".

         How a piece of code actually goes about doing its job.
         Users of the code should not count on implementation
         details staying the same unless they are part of the
         published "interface".

         To gain access to symbols that are exported from another
         module.  See "use" in perlfunc.

         To increase the value of something by 1 (or by some
         other number, if so specified).

         In olden days, the act of looking up a "key" in an
         actual index (such as a phone book), but now merely the
         act of using any kind of key or position to find the
         corresponding "value", even if no index is involved.
         Things have degenerated to the point that Perl's index

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         function merely locates the position (index) of one
         string in another.

     indirect filehandle
         An "expression" that evaluates to something that can be
         used as a "filehandle": a "string" (filehandle name), a
         "typeglob", a typeglob "reference", or a low-level "IO"

     indirect object
         In English grammar, a short noun phrase between a verb
         and its direct object indicating the beneficiary or
         recipient of the action.  In Perl, "print STDOUT
         "$foo\n";" can be understood as "verb indirect-object
         object" where "STDOUT" is the recipient of the print
         action, and "$foo" is the object being printed.
         Similarly, when invoking a "method", you might place the
         invocant between the method and its arguments:

           $gollum = new Pathetic::Creature "Smeagol";
           give $gollum "Fisssssh!";
           give $gollum "Precious!";

         In modern Perl, calling methods this way is often
         considered bad practice and to be avoided.

     indirect object slot
         The syntactic position falling between a method call and
         its arguments when using the indirect object invocation
         syntax.  (The slot is distinguished by the absence of a
         comma between it and the next argument.) "STDERR" is in
         the indirect object slot here:

           print STDERR "Awake!  Awake!  Fear, Fire,
               Foes!  Awake!\n";

         If something in a program isn't the value you're looking
         for but indicates where the value is, that's
         indirection.  This can be done with either symbolic
         references or hard references.

         An "operator" that comes in between its operands, such
         as multiplication in "24 * 7".

         What you get from your ancestors, genetically or
         otherwise.  If you happen to be a "class", your
         ancestors are called base classes and your descendants
         are called derived classes.  See "single inheritance"
         and "multiple inheritance".

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         Short for "an instance of a class", meaning an "object"
         of that "class".

     instance variable
         An "attribute" of an "object"; data stored with the
         particular object rather than with the class as a whole.

         A number with no fractional (decimal) part.  A counting
         number, like 1, 2, 3, and so on, but including 0 and the

         The services a piece of code promises to provide
         forever, in contrast to its "implementation", which it
         should feel free to change whenever it likes.

         The insertion of a scalar or list value somewhere in the
         middle of another value, such that it appears to have
         been there all along.  In Perl, variable interpolation
         happens in double-quoted strings and patterns, and list
         interpolation occurs when constructing the list of
         values to pass to a list operator or other such
         construct that takes a "LIST".

         Strictly speaking, a program that reads a second program
         and does what the second program says directly without
         turning the program into a different form first, which
         is what compilers do.  Perl is not an interpreter by
         this definition, because it contains a kind of compiler
         that takes a program and turns it into a more executable
         form (syntax trees) within the perl process itself,
         which the Perl "run time" system then interprets.

         The agent on whose behalf a "method" is invoked.  In a
         "class" method, the invocant is a package name.  In an
         "instance" method, the invocant is an object reference.

         The act of calling up a deity, daemon, program, method,
         subroutine, or function to get it do what you think it's
         supposed to do.  We usually "call" subroutines but
         "invoke" methods, since it sounds cooler.

     I/O Input from, or output to, a "file" or "device".

     IO  An internal I/O object.  Can also mean "indirect

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     IP  Internet Protocol, or Intellectual Property.

     IPC Interprocess Communication.

         A relationship between two objects in which one object
         is considered to be a more specific version of the
         other, generic object: "A camel is a mammal."  Since the
         generic object really only exists in a Platonic sense,
         we usually add a little abstraction to the notion of
         objects and think of the relationship as being between a
         generic "base class" and a specific "derived class".
         Oddly enough, Platonic classes don't always have
         Platonic relationships--see "inheritance".

         Doing something repeatedly.

         A special programming gizmo that keeps track of where
         you are in something that you're trying to iterate over.
         The "foreach" loop in Perl contains an iterator; so does
         a hash, allowing you to each through it.

     IV  The integer four, not to be confused with six, Tom's
         favorite editor.  IV also means an internal Integer
         Value of the type a "scalar" can hold, not to be
         confused with an "NV".

         "Just Another Perl Hacker," a clever but cryptic bit of
         Perl code that when executed, evaluates to that string.
         Often used to illustrate a particular Perl feature, and
         something of an ongoing Obfuscated Perl Contest seen in
         Usenix signatures.

     key The string index to a "hash", used to look up the
         "value" associated with that key.

         See "reserved words".

         A name you give to a "statement" so that you can talk
         about that statement elsewhere in the program.

         The quality that makes you go to great effort to reduce
         overall energy expenditure.  It makes you write labor-

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         saving programs that other people will find useful, and
         document what you wrote so you don't have to answer so
         many questions about it.  Hence, the first great virtue
         of a programmer.  Also hence, this book.  See also
         "impatience" and "hubris".

     left shift
         A "bit shift" that multiplies the number by some power
         of 2.

     leftmost longest
         The preference of the "regular expression" engine to
         match the leftmost occurrence of a "pattern", then given
         a position at which a match will occur, the preference
         for the longest match (presuming the use of a "greedy"
         quantifier).  See perlre for much more on this subject.

         Fancy term for a "token".

         Fancy term for a "tokener".

     lexical analysis
         Fancy term for "tokenizing".

     lexical scoping
         Looking at your Oxford English Dictionary through a
         microscope.  (Also known as "static scoping", because
         dictionaries don't change very fast.)  Similarly,
         looking at variables stored in a private dictionary
         (namespace) for each scope, which are visible only from
         their point of declaration down to the end of the
         lexical scope in which they are declared.  --Syn.
         "static scoping".  --Ant. "dynamic scoping".

     lexical variable
         A "variable" subject to "lexical scoping", declared by
         my.  Often just called a "lexical".  (The our
         declaration declares a lexically scoped name for a
         global variable, which is not itself a lexical

         Generally, a collection of procedures.  In ancient days,
         referred to a collection of subroutines in a .pl file.
         In modern times, refers more often to the entire
         collection of Perl modules on your system.

         Last In, First Out.  See also "FIFO".  A LIFO is usually
         called a "stack".

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         In Unix, a sequence of zero or more non-newline
         characters terminated with a "newline" character.  On
         non-Unix machines, this is emulated by the C library
         even if the underlying "operating system" has different

     line buffering
         Used by a "standard I/O" output stream that flushes its
         "buffer" after every "newline".  Many standard I/O
         libraries automatically set up line buffering on output
         that is going to the terminal.

     line number
         The number of lines read previous to this one, plus 1.
         Perl keeps a separate line number for each source or
         input file it opens.  The current source file's line
         number is represented by "__LINE__".  The current input
         line number (for the file that was most recently read
         via "<FH>") is represented by the $.
         ($INPUT_LINE_NUMBER) variable.  Many error messages
         report both values, if available.

         Used as a noun, a name in a "directory", representing a
         "file".  A given file can have multiple links to it.
         It's like having the same phone number listed in the
         phone directory under different names.  As a verb, to
         resolve a partially compiled file's unresolved symbols
         into a (nearly) executable image.  Linking can generally
         be static or dynamic, which has nothing to do with
         static or dynamic scoping.

         A syntactic construct representing a comma-separated
         list of expressions, evaluated to produce a "list
         value".  Each "expression" in a "LIST" is evaluated in
         "list context" and interpolated into the list value.

         An ordered set of scalar values.

     list context
         The situation in which an "expression" is expected by
         its surroundings (the code calling it) to return a list
         of values rather than a single value.  Functions that
         want a "LIST" of arguments tell those arguments that
         they should produce a list value.  See also "context".

     list operator
         An "operator" that does something with a list of values,
         such as join or grep.  Usually used for named built-in

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         operators (such as print, unlink, and system) that do
         not require parentheses around their "argument" list.

     list value
         An unnamed list of temporary scalar values that may be
         passed around within a program from any list-generating
         function to any function or construct that provides a
         "list context".

         A token in a programming language such as a number or
         "string" that gives you an actual "value" instead of
         merely representing possible values as a "variable"

         From Swift: someone who eats eggs little end first.
         Also used of computers that store the least significant
         "byte" of a word at a lower byte address than the most
         significant byte.  Often considered superior to big-
         endian machines.  See also "big-endian".

         Not meaning the same thing everywhere.  A global
         variable in Perl can be localized inside a dynamic scope
         via the local operator.

     logical operator
         Symbols representing the concepts "and", "or", "xor",
         and "not".

         An "assertion" that peeks at the string to the right of
         the current match location.

         An "assertion" that peeks at the string to the left of
         the current match location.

         A construct that performs something repeatedly, like a
         roller coaster.

     loop control statement
         Any statement within the body of a loop that can make a
         loop prematurely stop looping or skip an "iteration".
         Generally you shouldn't try this on roller coasters.

     loop label
         A kind of key or name attached to a loop (or roller
         coaster) so that loop control statements can talk about
         which loop they want to control.

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         Able to serve as an "lvalue".

         Term used by language lawyers for a storage location you
         can assign a new "value" to, such as a "variable" or an
         element of an "array".  The "l" is short for "left", as
         in the left side of an assignment, a typical place for
         lvalues.  An "lvaluable" function or expression is one
         to which a value may be assigned, as in "pos($x) = 10".

     lvalue modifier
         An adjectival pseudofunction that warps the meaning of
         an "lvalue" in some declarative fashion.  Currently
         there are three lvalue modifiers: my, our, and local.

         Technically speaking, any extra semantics attached to a
         variable such as $!, $0, %ENV, or %SIG, or to any tied
         variable.  Magical things happen when you diddle those

     magical increment
         An "increment" operator that knows how to bump up
         alphabetics as well as numbers.

     magical variables
         Special variables that have side effects when you access
         them or assign to them.  For example, in Perl, changing
         elements of the %ENV array also changes the
         corresponding environment variables that subprocesses
         will use.  Reading the $! variable gives you the current
         system error number or message.

         A file that controls the compilation of a program.  Perl
         programs don't usually need a "Makefile" because the
         Perl compiler has plenty of self-control.

     man The Unix program that displays online documentation
         (manual pages) for you.

         A "page" from the manuals, typically accessed via the
         man(1) command.  A manpage contains a SYNOPSIS, a
         DESCRIPTION, a list of BUGS, and so on, and is typically
         longer than a page.  There are manpages documenting
         commands, syscalls, "library" functions, devices,
         protocols, files, and such.  In this book, we call any
         piece of standard Perl documentation (like perlop or
         perldelta) a manpage, no matter what format it's

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         installed in on your system.

         See "pattern matching".

     member data
         See "instance variable".

         This always means your main memory, not your disk.
         Clouding the issue is the fact that your machine may
         implement "virtual" memory; that is, it will pretend
         that it has more memory than it really does, and it'll
         use disk space to hold inactive bits.  This can make it
         seem like you have a little more memory than you really
         do, but it's not a substitute for real memory.  The best
         thing that can be said about virtual memory is that it
         lets your performance degrade gradually rather than
         suddenly when you run out of real memory.  But your
         program can die when you run out of virtual memory too,
         if you haven't thrashed your disk to death first.

         A "character" that is not supposed to be treated
         normally.  Which characters are to be treated specially
         as metacharacters varies greatly from context to
         context.  Your "shell" will have certain metacharacters,
         double-quoted Perl strings have other metacharacters,
         and "regular expression" patterns have all the double-
         quote metacharacters plus some extra ones of their own.

         Something we'd call a "metacharacter" except that it's a
         sequence of more than one character.  Generally, the
         first character in the sequence must be a true
         metacharacter to get the other characters in the
         metasymbol to misbehave along with it.

         A kind of action that an "object" can take if you tell
         it to.  See perlobj.

         The belief that "small is beautiful."  Paradoxically, if
         you say something in a small language, it turns out big,
         and if you say it in a big language, it turns out small.
         Go figure.

         In the context of the stat syscall, refers to the field
         holding the "permission bits" and the type of the

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         See "statement modifier", "regular expression modifier",
         and "lvalue modifier", not necessarily in that order.

         A "file" that defines a "package" of (almost) the same
         name, which can either "export" symbols or function as
         an "object" class.  (A module's main .pm file may also
         load in other files in support of the module.)  See the
         use built-in.

         An integer divisor when you're interested in the
         remainder instead of the quotient.

         Short for Perl Monger, a purveyor of Perl.

         A temporary value scheduled to die when the current
         statement finishes.

     multidimensional array
         An array with multiple subscripts for finding a single
         element.  Perl implements these using references--see
         perllol and perldsc.

     multiple inheritance
         The features you got from your mother and father, mixed
         together unpredictably.  (See also "inheritance", and
         "single inheritance".)  In computer languages (including
         Perl), the notion that a given class may have multiple
         direct ancestors or base classes.

     named pipe
         A "pipe" with a name embedded in the "filesystem" so
         that it can be accessed by two unrelated processes.

         A domain of names.  You needn't worry about whether the
         names in one such domain have been used in another.  See

     network address
         The most important attribute of a socket, like your
         telephone's telephone number.  Typically an IP address.
         See also "port".

         A single character that represents the end of a line,
         with the ASCII value of 012 octal under Unix (but 015 on

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         a Mac), and represented by "\n" in Perl strings.  For
         Windows machines writing text files, and for certain
         physical devices like terminals, the single newline gets
         automatically translated by your C library into a line
         feed and a carriage return, but normally, no translation
         is done.

     NFS Network File System, which allows you to mount a remote
         filesystem as if it were local.

     null character
         A character with the ASCII value of zero.  It's used by
         C to terminate strings, but Perl allows strings to
         contain a null.

     null list
         A "list value" with zero elements, represented in Perl
         by "()".

     null string
         A "string" containing no characters, not to be confused
         with a string containing a "null character", which has a
         positive length and is "true".

     numeric context
         The situation in which an expression is expected by its
         surroundings (the code calling it) to return a number.
         See also "context" and "string context".

     NV  Short for Nevada, no part of which will ever be confused
         with civilization.  NV also means an internal floating-
         point Numeric Value of the type a "scalar" can hold, not
         to be confused with an "IV".

         Half a "byte", equivalent to one "hexadecimal" digit,
         and worth four bits.

         An "instance" of a "class".  Something that "knows" what
         user-defined type (class) it is, and what it can do
         because of what class it is.  Your program can request
         an object to do things, but the object gets to decide
         whether it wants to do them or not.  Some objects are
         more accommodating than others.

         A number in base 8.  Only the digits 0 through 7 are
         allowed.  Octal constants in Perl start with 0, as in
         013.  See also the oct function.

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         How many things you have to skip over when moving from
         the beginning of a string or array to a specific
         position within it.  Thus, the minimum offset is zero,
         not one, because you don't skip anything to get to the
         first item.

         An entire computer program crammed into one line of

     open source software
         Programs for which the source code is freely available
         and freely redistributable, with no commercial strings
         attached.  For a more detailed definition, see

         An "expression" that yields a "value" that an "operator"
         operates on.  See also "precedence".

     operating system
         A special program that runs on the bare machine and
         hides the gory details of managing processes and
         devices.  Usually used in a looser sense to indicate a
         particular culture of programming.  The loose sense can
         be used at varying levels of specificity.  At one
         extreme, you might say that all versions of Unix and
         Unix-lookalikes are the same operating system (upsetting
         many people, especially lawyers and other advocates).
         At the other extreme, you could say this particular
         version of this particular vendor's operating system is
         different from any other version of this or any other
         vendor's operating system.  Perl is much more portable
         across operating systems than many other languages.  See
         also "architecture" and "platform".

         A gizmo that transforms some number of input values to
         some number of output values, often built into a
         language with a special syntax or symbol.  A given
         operator may have specific expectations about what types
         of data you give as its arguments (operands) and what
         type of data you want back from it.

     operator overloading
         A kind of "overloading" that you can do on built-in
         operators to make them work on objects as if the objects
         were ordinary scalar values, but with the actual
         semantics supplied by the object class.  This is set up
         with the overload "pragma".

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         See either switches or "regular expression modifier".

         Giving additional meanings to a symbol or construct.
         Actually, all languages do overloading to one extent or
         another, since people are good at figuring out things
         from "context".

         Hiding or invalidating some other definition of the same
         name.  (Not to be confused with "overloading", which
         adds definitions that must be disambiguated some other
         way.) To confuse the issue further, we use the word with
         two overloaded definitions: to describe how you can
         define your own "subroutine" to hide a built-in
         "function" of the same name (see "Overriding Built-in
         Functions" in perlsub) and to describe how you can
         define a replacement "method" in a "derived class" to
         hide a "base class"'s method of the same name (see

         The one user (apart from the superuser) who has absolute
         control over a "file".  A file may also have a "group"
         of users who may exercise joint ownership if the real
         owner permits it.  See "permission bits".

         A "namespace" for global variables, subroutines, and the
         like, such that they can be kept separate from like-
         named symbols in other namespaces.  In a sense, only the
         package is global, since the symbols in the package's
         symbol table are only accessible from code compiled
         outside the package by naming the package.  But in
         another sense, all package symbols are also
         globals--they're just well-organized globals.

     pad Short for "scratchpad".

         See "argument".

     parent class
         See "base class".

     parse tree
         See "syntax tree".

         The subtle but sometimes brutal art of attempting to

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         turn your possibly malformed program into a valid
         "syntax tree".

         To fix by applying one, as it were.  In the realm of
         hackerdom, a listing of the differences between two
         versions of a program as might be applied by the
         patch(1) program when you want to fix a bug or upgrade
         your old version.

         The list of directories the system searches to find a
         program you want to "execute".  The list is stored as
         one of your environment variables, accessible in Perl as

         A fully qualified filename such as /usr/bin/perl.
         Sometimes confused with "PATH".

         A template used in "pattern matching".

     pattern matching
         Taking a pattern, usually a "regular expression", and
         trying the pattern various ways on a string to see
         whether there's any way to make it fit.  Often used to
         pick interesting tidbits out of a file.

     permission bits
         Bits that the "owner" of a file sets or unsets to allow
         or disallow access to other people.  These flag bits are
         part of the "mode" word returned by the stat built-in
         when you ask about a file.  On Unix systems, you can
         check the ls(1) manpage for more information.

         What you get when you do "Perl++" twice.  Doing it only
         once will curl your hair.  You have to increment it
         eight times to shampoo your hair.  Lather, rinse,

         A direct "connection" that carries the output of one
         "process" to the input of another without an
         intermediate temporary file.  Once the pipe is set up,
         the two processes in question can read and write as if
         they were talking to a normal file, with some caveats.

         A series of processes all in a row, linked by pipes,
         where each passes its output stream to the next.

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         The entire hardware and software context in which a
         program runs.  A
          program written in a platform-dependent language might
         break if you change any of: machine, operating system,
         libraries, compiler, or system configuration.  The perl
         interpreter has to be compiled differently for each
         platform because it is implemented in C, but programs
         written in the Perl language are largely platform-

     pod The markup used to embed documentation into your Perl
         code.  See perlpod.

         A "variable" in a language like C that contains the
         exact memory location of some other item.  Perl handles
         pointers internally so you don't have to worry about
         them.  Instead, you just use symbolic pointers in the
         form of keys and "variable" names, or hard references,
         which aren't pointers (but act like pointers and do in
         fact contain pointers).

         The notion that you can tell an "object" to do something
         generic, and the object will interpret the command in
         different ways depending on its type.  [<Gk many shapes]

         The part of the address of a TCP or UDP socket that
         directs packets to the correct process after finding the
         right machine, something like the phone extension you
         give when you reach the company operator.  Also, the
         result of converting code to run on a different platform
         than originally intended, or the verb denoting this

         Once upon a time, C code compilable under both BSD and
         SysV.  In general, code that can be easily converted to
         run on another "platform", where "easily" can be defined
         however you like, and usually is.  Anything may be
         considered portable if you try hard enough.  See mobile
         home or London Bridge.

         Someone who "carries" software from one "platform" to
         another.  Porting programs written in platform-dependent
         languages such as C can be difficult work, but porting
         programs like Perl is very much worth the agony.


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         The Portable Operating System Interface specification.

         An "operator" that follows its "operand", as in "$x++".

     pp  An internal shorthand for a "push-pop" code, that is, C
         code implementing Perl's stack machine.

         A standard module whose practical hints and suggestions
         are received (and possibly ignored) at compile time.
         Pragmas are named in all lowercase.

         The rules of conduct that, in the absence of other
         guidance, determine what should happen first.  For
         example, in the absence of parentheses, you always do
         multiplication before addition.

         An "operator" that precedes its "operand", as in "++$x".

         What some helper "process" did to transform the incoming
         data into a form more suitable for the current process.
         Often done with an incoming "pipe".  See also "C

         A "subroutine".

         An instance of a running program.  Under multitasking
         systems like Unix, two or more separate processes could
         be running the same program independently at the same
         time--in fact, the fork function is designed to bring
         about this happy state of affairs.  Under other
         operating systems, processes are sometimes called
         "threads", "tasks", or "jobs", often with slight nuances
         in meaning.

     program generator
         A system that algorithmically writes code for you in a
         high-level language.  See also "code generator".

     progressive matching
         Pattern matching that picks up where it left off before.

         See either "instance variable" or "character property".


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         In networking, an agreed-upon way of sending messages
         back and forth so that neither correspondent will get
         too confused.

         An optional part of a "subroutine" declaration telling
         the Perl compiler how many and what flavor of arguments
         may be passed as "actual arguments", so that you can
         write subroutine calls that parse much like built-in
         functions.  (Or don't parse, as the case may be.)

         A construct that sometimes looks like a function but
         really isn't.  Usually reserved for "lvalue" modifiers
         like my, for "context" modifiers like scalar, and for
         the pick-your-own-quotes constructs, "q//", "qq//",
         "qx//", "qw//", "qr//", "m//", "s///", "y///", and

         A reference to an array whose initial element happens to
         hold a reference to a hash.  You can treat a pseudohash
         reference as either an array reference or a hash

         An "operator" that looks something like a "literal",
         such as the output-grabbing operator, "`""command""`".

     public domain
         Something not owned by anybody.  Perl is copyrighted and
         is thus not in the public domain--it's just "freely
         available" and "freely redistributable".

         A notional "baton" handed around the Perl community
         indicating who is the lead integrator in some arena of

         A "pumpkin" holder, the person in charge of pumping the
         pump, or at least priming it.  Must be willing to play
         the part of the Great Pumpkin now and then.

     PV  A "pointer value", which is Perl Internals Talk for a

         Possessing a complete name.  The symbol $Ent::moot is
         qualified; $moot is unqualified.  A fully qualified
         filename is specified from the top-level directory.

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         A component of a "regular expression" specifying how
         many times the foregoing "atom" may occur.

         With respect to files, one that has the proper
         permission bit set to let you access the file.  With
         respect to computer programs, one that's written well
         enough that someone has a chance of figuring out what
         it's trying to do.

         The last rites performed by a parent "process" on behalf
         of a deceased child process so that it doesn't remain a
         "zombie".  See the wait and waitpid function calls.

         A set of related data values in a "file" or "stream",
         often associated with a unique "key" field.  In Unix,
         often commensurate with a "line", or a blank-line-
         terminated set of lines (a "paragraph").  Each line of
         the /etc/passwd file is a record, keyed on login name,
         containing information about that user.

         The art of defining something (at least partly) in terms
         of itself, which is a naughty no-no in dictionaries but
         often works out okay in computer programs if you're
         careful not to recurse forever, which is like an
         infinite loop with more spectacular failure modes.

         Where you look to find a pointer to information
         somewhere else.  (See "indirection".)  References come
         in two flavors, symbolic references and hard references.

         Whatever a reference refers to, which may or may not
         have a name.  Common types of referents include scalars,
         arrays, hashes, and subroutines.

         See "regular expression".

     regular expression
         A single entity with various interpretations, like an
         elephant.  To a computer scientist, it's a grammar for a
         little language in which some strings are legal and
         others aren't.  To normal people, it's a pattern you can
         use to find what you're looking for when it varies from
         case to case.  Perl's regular expressions are far from

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         regular in the theoretical sense, but in regular use
         they work quite well.  Here's a regular expression: "/Oh
         s.*t./".  This will match strings like ""Oh say can you
         see by the dawn's early light"" and ""Oh sit!"".  See

     regular expression modifier
         An option on a pattern or substitution, such as "/i" to
         render the pattern case insensitive.  See also

     regular file
         A "file" that's not a "directory", a "device", a named
         "pipe" or "socket", or a "symbolic link".  Perl uses the
         "-f" file test operator to identify regular files.
         Sometimes called a "plain" file.

     relational operator
         An "operator" that says whether a particular ordering
         relationship is "true" about a pair of operands.  Perl
         has both numeric and string relational operators.  See
         "collating sequence".

     reserved words
         A word with a specific, built-in meaning to a
         "compiler", such as "if" or delete.  In many languages
         (not Perl), it's illegal to use reserved words to name
         anything else.  (Which is why they're reserved, after
         all.)  In Perl, you just can't use them to name labels
         or filehandles.  Also called "keywords".

     return value
         The "value" produced by a "subroutine" or "expression"
         when evaluated.  In Perl, a return value may be either a
         "list" or a "scalar".

     RFC Request For Comment, which despite the timid
         connotations is the name of a series of important
         standards documents.

     right shift
         A "bit shift" that divides a number by some power of 2.

         The superuser (UID == 0).  Also, the top-level directory
         of the filesystem.

         What you are told when someone thinks you should Read
         The Fine Manual.

     run phase

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         Any time after Perl starts running your main program.
         See also "compile phase".  Run phase is mostly spent in
         "run time" but may also be spent in "compile time" when
         require, do "FILE", or eval "STRING" operators are
         executed or when a substitution uses the "/ee" modifier.

     run time
         The time when Perl is actually doing what your code says
         to do, as opposed to the earlier period of time when it
         was trying to figure out whether what you said made any
         sense whatsoever, which is "compile time".

     run-time pattern
         A pattern that contains one or more variables to be
         interpolated before parsing the pattern as a "regular
         expression", and that therefore cannot be analyzed at
         compile time, but must be re-analyzed each time the
         pattern match operator is evaluated.  Run-time patterns
         are useful but expensive.

     RV  A recreational vehicle, not to be confused with
         vehicular recreation.  RV also means an internal
         Reference Value of the type a "scalar" can hold.  See
         also "IV" and "NV" if you're not confused yet.

         A "value" that you might find on the right side of an
         "assignment".  See also "lvalue".

         A simple, singular value; a number, "string", or

     scalar context
         The situation in which an "expression" is expected by
         its surroundings (the code calling it) to return a
         single "value" rather than a "list" of values.  See also
         "context" and "list context".  A scalar context
         sometimes imposes additional constraints on the return
         value--see "string context" and "numeric context".
         Sometimes we talk about a "Boolean context" inside
         conditionals, but this imposes no additional
         constraints, since any scalar value, whether numeric or
         "string", is already true or false.

     scalar literal
         A number or quoted "string"--an actual "value" in the
         text of your program, as opposed to a "variable".

     scalar value
         A value that happens to be a "scalar" as opposed to a

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     scalar variable
         A "variable" prefixed with "$" that holds a single

         How far away you can see a variable from, looking
         through one.  Perl has two visibility mechanisms: it
         does "dynamic scoping" of local variables, meaning that
         the rest of the "block", and any subroutines that are
         called by the rest of the block, can see the variables
         that are local to the block.  Perl does "lexical
         scoping" of my variables, meaning that the rest of the
         block can see the variable, but other subroutines called
         by the block cannot see the variable.

         The area in which a particular invocation of a
         particular file or subroutine keeps some of its
         temporary values, including any lexically scoped

         A text "file" that is a program intended to be executed
         directly rather than compiled to another form of file
         before execution.  Also, in the context of "Unicode", a
         writing system for a particular language or group of
         languages, such as Greek, Bengali, or Klingon.

     script kiddie
         A "cracker" who is not a "hacker", but knows just enough
         to run canned scripts.  A cargo-cult programmer.

     sed A venerable Stream EDitor from which Perl derives some
         of its ideas.

         A fancy kind of interlock that prevents multiple threads
         or processes from using up the same resources

         A "character" or "string" that keeps two surrounding
         strings from being confused with each other.  The split
         function works on separators.  Not to be confused with
         delimiters or terminators.  The "or" in the previous
         sentence separated the two alternatives.

         Putting a fancy "data structure" into linear order so
         that it can be stored as a "string" in a disk file or

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         database or sent through a "pipe".  Also called

         In networking, a "process" that either advertises a
         "service" or just hangs around at a known location and
         waits for clients who need service to get in touch with

         Something you do for someone else to make them happy,
         like giving them the time of day (or of their life).  On
         some machines, well-known services are listed by the
         getservent function.

         Same as "setuid", only having to do with giving away
         "group" privileges.

         Said of a program that runs with the privileges of its
         "owner" rather than (as is usually the case) the
         privileges of whoever is running it.  Also describes the
         bit in the mode word ("permission bits") that controls
         the feature.  This bit must be explicitly set by the
         owner to enable this feature, and the program must be
         carefully written not to give away more privileges than
         it ought to.

     shared memory
         A piece of "memory" accessible by two different
         processes who otherwise would not see each other's

         Irish for the whole McGillicuddy.  In Perl culture, a
         portmanteau of "sharp" and "bang", meaning the "#!"
         sequence that tells the system where to find the

         A "command"-line "interpreter".  The program that
         interactively gives you a prompt, accepts one or more
         lines of input, and executes the programs you mentioned,
         feeding each of them their proper arguments and input
         data.  Shells can also execute scripts containing such
         commands.  Under Unix, typical shells include the Bourne
         shell (/bin/sh), the C shell (/bin/csh), and the Korn
         shell (/bin/ksh).  Perl is not strictly a shell because
         it's not interactive (although Perl programs can be

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     side effects
         Something extra that happens when you evaluate an
         "expression".  Nowadays it can refer to almost anything.
         For example, evaluating a simple assignment statement
         typically has the "side effect" of assigning a value to
         a variable.  (And you thought assigning the value was
         your primary intent in the first place!)  Likewise,
         assigning a value to the special variable $|
         ($AUTOFLUSH) has the side effect of forcing a flush
         after every write or print on the currently selected

         A bolt out of the blue; that is, an event triggered by
         the "operating system", probably when you're least
         expecting it.

     signal handler
         A "subroutine" that, instead of being content to be
         called in the normal fashion, sits around waiting for a
         bolt out of the blue before it will deign to "execute".
         Under Perl, bolts out of the blue are called signals,
         and you send them with the kill built-in.  See "%SIG" in
         perlvar and "Signals" in perlipc.

     single inheritance
         The features you got from your mother, if she told you
         that you don't have a father.  (See also "inheritance"
         and "multiple inheritance".)  In computer languages, the
         notion that classes reproduce asexually so that a given
         class can only have one direct ancestor or "base class".
         Perl supplies no such restriction, though you may
         certainly program Perl that way if you like.

         A selection of any number of elements from a "list",
         "array", or "hash".

         To read an entire "file" into a "string" in one

         An endpoint for network communication among multiple
         processes that works much like a telephone or a post
         office box.  The most important thing about a socket is
         its "network address" (like a phone number).  Different
         kinds of sockets have different kinds of addresses--some
         look like filenames, and some don't.

     soft reference
         See "symbolic reference".

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     source filter
         A special kind of "module" that does "preprocessing" on
         your script just before it gets to the "tokener".

         A device you can put things on the top of, and later
         take them back off in the opposite order in which you
         put them on.  See "LIFO".

         Included in the official Perl distribution, as in a
         standard module, a standard tool, or a standard Perl

     standard error
         The default output "stream" for nasty remarks that don't
         belong in "standard output".  Represented within a Perl
         program by the "filehandle" "STDERR".  You can use this
         stream explicitly, but the die and warn built-ins write
         to your standard error stream automatically.

     standard I/O
         A standard C library for doing buffered input and output
         to the "operating system".  (The "standard" of standard
         I/O is only marginally related to the "standard" of
         standard input and output.)  In general, Perl relies on
         whatever implementation of standard I/O a given
         operating system supplies, so the buffering
         characteristics of a Perl program on one machine may not
         exactly match those on another machine.  Normally this
         only influences efficiency, not semantics.  If your
         standard I/O package is doing block buffering and you
         want it to "flush" the buffer more often, just set the
         $| variable to a true value.

     standard input
         The default input "stream" for your program, which if
         possible shouldn't care where its data is coming from.
         Represented within a Perl program by the "filehandle"

     standard output
         The default output "stream" for your program, which if
         possible shouldn't care where its data is going.
         Represented within a Perl program by the "filehandle"

     stat structure
         A special internal spot in which Perl keeps the
         information about the last "file" on which you requested

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         A "command" to the computer about what to do next, like
         a step in a recipe: "Add marmalade to batter and mix
         until mixed."  A statement is distinguished from a
         "declaration", which doesn't tell the computer to do
         anything, but just to learn something.

     statement modifier
         A "conditional" or "loop" that you put after the
         "statement" instead of before, if you know what we mean.

         Varying slowly compared to something else.
         (Unfortunately, everything is relatively stable compared
         to something else, except for certain elementary
         particles, and we're not so sure about them.)  In
         computers, where things are supposed to vary rapidly,
         "static" has a derogatory connotation, indicating a
         slightly dysfunctional "variable", "subroutine", or
         "method".  In Perl culture, the word is politely

     static method
         No such thing.  See "class method".

     static scoping
         No such thing.  See "lexical scoping".

     static variable
         No such thing.  Just use a "lexical variable" in a scope
         larger than your "subroutine".

         The "value" returned to the parent "process" when one of
         its child processes dies.  This value is placed in the
         special variable $?.  Its upper eight bits are the exit
         status of the defunct process, and its lower eight bits
         identify the signal (if any) that the process died from.
         On Unix systems, this status value is the same as the
         status word returned by wait(2).  See "system" in

         See "standard error".

         See "standard input".

         See "standard I/O".


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         See "standard output".

         A flow of data into or out of a process as a steady
         sequence of bytes or characters, without the appearance
         of being broken up into packets.  This is a kind of
         "interface"--the underlying "implementation" may well
         break your data up into separate packets for delivery,
         but this is hidden from you.

         A sequence of characters such as "He said !@#*&%@#*?!".
         A string does not have to be entirely printable.

     string context
         The situation in which an expression is expected by its
         surroundings (the code calling it) to return a "string".
         See also "context" and "numeric context".

         The process of producing a "string" representation of an
         abstract object.

         C keyword introducing a structure definition or name.

         See "data structure".

         See "derived class".

         A component of a "regular expression" pattern.

         A named or otherwise accessible piece of program that
         can be invoked from elsewhere in the program in order to
         accomplish some sub-goal of the program.  A subroutine
         is often parameterized to accomplish different but
         related things depending on its input arguments.  If the
         subroutine returns a meaningful "value", it is also
         called a "function".

         A "value" that indicates the position of a particular
         "array" "element" in an array.

         Changing parts of a string via the "s///" operator.  (We
         avoid use of this term to mean "variable

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         A portion of a "string", starting at a certain
         "character" position ("offset") and proceeding for a
         certain number of characters.

         See "base class".

         The person whom the "operating system" will let do
         almost anything.  Typically your system administrator or
         someone pretending to be your system administrator.  On
         Unix systems, the "root" user.  On Windows systems,
         usually the Administrator user.

     SV  Short for "scalar value".  But within the Perl
         interpreter every "referent" is treated as a member of a
         class derived from SV, in an object-oriented sort of
         way.  Every "value" inside Perl is passed around as a C
         language "SV*" pointer.  The SV "struct" knows its own
         "referent type", and the code is smart enough (we hope)
         not to try to call a "hash" function on a "subroutine".

         An option you give on a command line to influence the
         way your program works, usually introduced with a minus
         sign.  The word is also used as a nickname for a "switch

     switch cluster
         The combination of multiple command-line switches (e.g.,
         -a -b -c) into one switch (e.g., -abc).  Any switch with
         an additional "argument" must be the last switch in a

     switch statement
         A program technique that lets you evaluate an
         "expression" and then, based on the value of the
         expression, do a multiway branch to the appropriate
         piece of code for that value.  Also called a "case
         structure", named after the similar Pascal construct.
         Most switch statements in Perl are spelled "for".  See
         "Basic BLOCKs and Switch Statements" in perlsyn.

         Generally, any "token" or "metasymbol".  Often used more
         specifically to mean the sort of name you might find in
         a "symbol table".

     symbol table
         Where a "compiler" remembers symbols.  A program like
         Perl must somehow remember all the names of all the

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         variables, filehandles, and subroutines you've used.  It
         does this by placing the names in a symbol table, which
         is implemented in Perl using a "hash table".  There is a
         separate symbol table for each "package" to give each
         package its own "namespace".

     symbolic debugger
         A program that lets you step through the execution of
         your program, stopping or printing things out here and
         there to see whether anything has gone wrong, and if so,
         what.  The "symbolic" part just means that you can talk
         to the debugger using the same symbols with which your
         program is written.

     symbolic link
         An alternate filename that points to the real
         "filename", which in turn points to the real "file".
         Whenever the "operating system" is trying to parse a
         "pathname" containing a symbolic link, it merely
         substitutes the new name and continues parsing.

     symbolic reference
         A variable whose value is the name of another variable
         or subroutine.  By dereferencing the first variable, you
         can get at the second one.  Symbolic references are
         illegal under use strict 'refs'.

         Programming in which the orderly sequence of events can
         be determined; that is, when things happen one after the
         other, not at the same time.

     syntactic sugar
         An alternative way of writing something more easily; a

         From Greek, "with-arrangement".  How things
         (particularly symbols) are put together with each other.

     syntax tree
         An internal representation of your program wherein
         lower-level constructs dangle off the higher-level
         constructs enclosing them.

         A "function" call directly to the "operating system".
         Many of the important subroutines and functions you use
         aren't direct system calls, but are built up in one or
         more layers above the system call level.  In general,
         Perl programmers don't need to worry about the
         distinction.  However, if you do happen to know which

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         Perl functions are really syscalls, you can predict
         which of these will set the $!  ($ERRNO) variable on
         failure.  Unfortunately, beginning programmers often
         confusingly employ the term "system call" to mean what
         happens when you call the Perl system function, which
         actually involves many syscalls.  To avoid any
         confusion, we nearly always use say "syscall" for
         something you could call indirectly via Perl's syscall
         function, and never for something you would call with
         Perl's system function.

         Said of data derived from the grubby hands of a user and
         thus unsafe for a secure program to rely on.  Perl does
         taint checks if you run a "setuid" (or "setgid")
         program, or if you use the -T switch.

     TCP Short for Transmission Control Protocol.  A protocol
         wrapped around the Internet Protocol to make an
         unreliable packet transmission mechanism appear to the
         application program to be a reliable "stream" of bytes.

         Short for a "terminal", that is, a leaf node of a
         "syntax tree".  A thing that functions grammatically as
         an "operand" for the operators in an expression.

         A "character" or "string" that marks the end of another
         string.  The $/ variable contains the string that
         terminates a readline operation, which chomp deletes
         from the end.  Not to be confused with delimiters or
         separators.  The period at the end of this sentence is a

         An "operator" taking three operands.  Sometimes
         pronounced "trinary".

         A "string" or "file" containing primarily printable

         Like a forked process, but without "fork"'s inherent
         memory protection.  A thread is lighter weight than a
         full process, in that a process could have multiple
         threads running around in it, all fighting over the same
         process's memory space unless steps are taken to protect
         threads from each other.  See threads.

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     tie The bond between a magical variable and its
         implementation class.  See "tie" in perlfunc and

         There's More Than One Way To Do It, the Perl Motto.  The
         notion that there can be more than one valid path to
         solving a programming problem in context.  (This doesn't
         mean that more ways are always better or that all
         possible paths are equally desirable--just that there
         need not be One True Way.)  Pronounced TimToady.

         A morpheme in a programming language, the smallest unit
         of text with semantic significance.

         A module that breaks a program text into a sequence of
         tokens for later analysis by a parser.

         Splitting up a program text into tokens.  Also known as
         "lexing", in which case you get "lexemes" instead of

     toolbox approach
         The notion that, with a complete set of simple tools
         that work well together, you can build almost anything
         you want.  Which is fine if you're assembling a
         tricycle, but if you're building a defranishizing
         comboflux regurgalator, you really want your own machine
         shop in which to build special tools.  Perl is sort of a
         machine shop.

         To turn one string representation into another by
         mapping each character of the source string to its
         corresponding character in the result string.  See
         "tr/SEARCHLIST/REPLACEMENTLIST/cds" in perlop.

         An event that causes a "handler" to be run.

         Not a stellar system with three stars, but an "operator"
         taking three operands.  Sometimes pronounced "ternary".

         A venerable typesetting language from which Perl derives
         the name of its $% variable and which is secretly used
         in the production of Camel books.

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         Any scalar value that doesn't evaluate to 0 or "".

         Emptying a file of existing contents, either
         automatically when opening a file for writing or
         explicitly via the truncate function.

         See "data type" and "class".

     type casting
         Converting data from one type to another.  C permits
         this.  Perl does not need it.  Nor want it.

     typed lexical
         A "lexical variable" that is declared with a "class"
         type: "my Pony $bill".

         A type definition in the C language.

         Use of a single identifier, prefixed with "*".  For
         example, *name stands for any or all of $name, @name,
         %name, &name, or just "name".  How you use it determines
         whether it is interpreted as all or only one of them.
         See "Typeglobs and Filehandles" in perldata.

         A description of how C types may be transformed to and
         from Perl types within an "extension" module written in

     UDP User Datagram Protocol, the typical way to send
         datagrams over the Internet.

     UID A user ID.  Often used in the context of "file" or
         "process" ownership.

         A mask of those "permission bits" that should be forced
         off when creating files or directories, in order to
         establish a policy of whom you'll ordinarily deny access
         to.  See the umask function.

     unary operator
         An operator with only one "operand", like "!" or chdir.
         Unary operators are usually prefix operators; that is,
         they precede their operand.  The "++" and "--" operators
         can be either prefix or postfix.  (Their position does

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         change their meanings.)

         A character set comprising all the major character sets
         of the world, more or less.  See perlunicode and

         A very large and constantly evolving language with
         several alternative and largely incompatible syntaxes,
         in which anyone can define anything any way they choose,
         and usually do.  Speakers of this language think it's
         easy to learn because it's so easily twisted to one's
         own ends, but dialectical differences make tribal
         intercommunication nearly impossible, and travelers are
         often reduced to a pidgin-like subset of the language.
         To be universally understood, a Unix shell programmer
         must spend years of study in the art.  Many have
         abandoned this discipline and now communicate via an
         Esperanto-like language called Perl.

         In ancient times, Unix was also used to refer to some
         code that a couple of people at Bell Labs wrote to make
         use of a PDP-7 computer that wasn't doing much of
         anything else at the time.

         An actual piece of data, in contrast to all the
         variables, references, keys, indexes, operators, and
         whatnot that you need to access the value.

         A named storage location that can hold any of various
         kinds of "value", as your program sees fit.

     variable interpolation
         The "interpolation" of a scalar or array variable into a

         Said of a "function" that happily receives an
         indeterminate number of "actual arguments".

         Mathematical jargon for a list of scalar values.

         Providing the appearance of something without the
         reality, as in: virtual memory is not real memory.  (See
         also "memory".)  The opposite of "virtual" is
         "transparent", which means providing the reality of

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         something without the appearance, as in: Perl handles
         the variable-length UTF-8 character encoding

     void context
         A form of "scalar context" in which an "expression" is
         not expected to return any "value" at all and is
         evaluated for its "side effects" alone.

         A "version" or "vector" "string" specified with a "v"
         followed by a series of decimal integers in dot
         notation, for instance, "v1.20.300.4000".  Each number
         turns into a "character" with the specified ordinal
         value.  (The "v" is optional when there are at least
         three integers.)

         A message printed to the "STDERR" stream to the effect
         that something might be wrong but isn't worth blowing up
         over.  See "warn" in perlfunc and the warnings pragma.

     watch expression
         An expression which, when its value changes, causes a
         breakpoint in the Perl debugger.

         A "character" that moves your cursor but doesn't
         otherwise put anything on your screen.  Typically refers
         to any of: space, tab, line feed, carriage return, or
         form feed.

         In normal "computerese", the piece of data of the size
         most efficiently handled by your computer, typically 32
         bits or so, give or take a few powers of 2.  In Perl
         culture, it more often refers to an alphanumeric
         "identifier" (including underscores), or to a string of
         nonwhitespace characters bounded by whitespace or string

     working directory
         Your current "directory", from which relative pathnames
         are interpreted by the "operating system".  The
         operating system knows your current directory because
         you told it with a chdir or because you started out in
         the place where your parent "process" was when you were

         A program or subroutine that runs some other program or

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         subroutine for you, modifying some of its input or
         output to better suit your purposes.

         What You See Is What You Get.  Usually used when
         something that appears on the screen matches how it will
         eventually look, like Perl's format declarations.  Also
         used to mean the opposite of magic because everything
         works exactly as it appears, as in the three-argument
         form of open.

     XS  An extraordinarily exported, expeditiously excellent,
         expressly eXternal Subroutine, executed in existing C or
         C++ or in an exciting new extension language called
         (exasperatingly) XS.  Examine perlxs for the exact
         explanation or perlxstut for an exemplary unexacting

         An external "subroutine" defined in "XS".

         Yet Another Compiler Compiler.  A parser generator
         without which Perl probably would not have existed.  See
         the file perly.y in the Perl source distribution.

     zero width
         A subpattern "assertion" matching the "null string"
         between characters.

         A process that has died (exited) but whose parent has
         not yet received proper notification of its demise by
         virtue of having called wait or waitpid.  If you fork,
         you must clean up after your child processes when they
         exit, or else the process table will fill up and your
         system administrator will Not Be Happy with you.

     Based on the Glossary of Programming Perl, Third Edition, by
     Larry Wall, Tom Christiansen & Jon Orwant.  Copyright (c)
     2000, 1996, 1991 O'Reilly Media, Inc.  This document may be
     distributed under the same terms as Perl itself.

     See attributes(5) for descriptions of the following

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     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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