man pages section 1: User Commands

Exit Print View

Updated: July 2014

perldata (1)


perldata - Perl data types


Please see following description for synopsis


Perl Programmers Reference Guide                      PERLDATA(1)

     perldata - Perl data types

  Variable names
     Perl has three built-in data types: scalars, arrays of
     scalars, and associative arrays of scalars, known as
     "hashes".  A scalar is a single string (of any size, limited
     only by the available memory), number, or a reference to
     something (which will be discussed in perlref).  Normal
     arrays are ordered lists of scalars indexed by number,
     starting with 0.  Hashes are unordered collections of scalar
     values indexed by their associated string key.

     Values are usually referred to by name, or through a named
     reference.  The first character of the name tells you to
     what sort of data structure it refers.  The rest of the name
     tells you the particular value to which it refers.  Usually
     this name is a single identifier, that is, a string
     beginning with a letter or underscore, and containing
     letters, underscores, and digits.  In some cases, it may be
     a chain of identifiers, separated by "::" (or by the
     slightly archaic "'"); all but the last are interpreted as
     names of packages, to locate the namespace in which to look
     up the final identifier (see "Packages" in perlmod for
     details).  It's possible to substitute for a simple
     identifier, an expression that produces a reference to the
     value at runtime.   This is described in more detail below
     and in perlref.

     Perl also has its own built-in variables whose names don't
     follow these rules.  They have strange names so they don't
     accidentally collide with one of your normal variables.
     Strings that match parenthesized parts of a regular
     expression are saved under names containing only digits
     after the "$" (see perlop and perlre).  In addition, several
     special variables that provide windows into the inner
     working of Perl have names containing punctuation characters
     and control characters.  These are documented in perlvar.

     Scalar values are always named with '$', even when referring
     to a scalar that is part of an array or a hash.  The '$'
     symbol works semantically like the English word "the" in
     that it indicates a single value is expected.

         $days               # the simple scalar value "days"
         $days[28]           # the 29th element of array @days
         $days{'Feb'}        # the 'Feb' value from hash %days
         $#days              # the last index of array @days

     Entire arrays (and slices of arrays and hashes) are denoted
     by '@', which works much like the word "these" or "those"

perl v5.12.5         Last change: 2012-11-03                    1

Perl Programmers Reference Guide                      PERLDATA(1)

     does in English, in that it indicates multiple values are

         @days               # ($days[0], $days[1],... $days[n])
         @days[3,4,5]        # same as ($days[3],$days[4],$days[5])
         @days{'a','c'}      # same as ($days{'a'},$days{'c'})

     Entire hashes are denoted by '%':

         %days               # (key1, val1, key2, val2 ...)

     In addition, subroutines are named with an initial '&',
     though this is optional when unambiguous, just as the word
     "do" is often redundant in English.  Symbol table entries
     can be named with an initial '*', but you don't really care
     about that yet (if ever :-).

     Every variable type has its own namespace, as do several
     non-variable identifiers.  This means that you can, without
     fear of conflict, use the same name for a scalar variable,
     an array, or a hash--or, for that matter, for a filehandle,
     a directory handle, a subroutine name, a format name, or a
     label.  This means that $foo and @foo are two different
     variables.  It also means that $foo[1] is a part of @foo,
     not a part of $foo.  This may seem a bit weird, but that's
     okay, because it is weird.

     Because variable references always start with '$', '@', or
     '%', the "reserved" words aren't in fact reserved with
     respect to variable names.  They are reserved with respect
     to labels and filehandles, however, which don't have an
     initial special character.  You can't have a filehandle
     named "log", for instance.  Hint: you could say
     "open(LOG,'logfile')" rather than "open(log,'logfile')".
     Using uppercase filehandles also improves readability and
     protects you from conflict with future reserved words.  Case
     is significant--"FOO", "Foo", and "foo" are all different
     names.  Names that start with a letter or underscore may
     also contain digits and underscores.

     It is possible to replace such an alphanumeric name with an
     expression that returns a reference to the appropriate type.
     For a description of this, see perlref.

     Names that start with a digit may contain only more digits.
     Names that do not start with a letter, underscore, digit or
     a caret (i.e.  a control character) are limited to one
     character, e.g.,  $% or $$.  (Most of these one character
     names have a predefined significance to Perl.  For instance,
     $$ is the current process id.)

perl v5.12.5         Last change: 2012-11-03                    2

Perl Programmers Reference Guide                      PERLDATA(1)

     The interpretation of operations and values in Perl
     sometimes depends on the requirements of the context around
     the operation or value.  There are two major contexts: list
     and scalar.  Certain operations return list values in
     contexts wanting a list, and scalar values otherwise.  If
     this is true of an operation it will be mentioned in the
     documentation for that operation.  In other words, Perl
     overloads certain operations based on whether the expected
     return value is singular or plural.  Some words in English
     work this way, like "fish" and "sheep".

     In a reciprocal fashion, an operation provides either a
     scalar or a list context to each of its arguments.  For
     example, if you say

         int( <STDIN> )

     the integer operation provides scalar context for the <>
     operator, which responds by reading one line from STDIN and
     passing it back to the integer operation, which will then
     find the integer value of that line and return that.  If, on
     the other hand, you say

         sort( <STDIN> )

     then the sort operation provides list context for <>, which
     will proceed to read every line available up to the end of
     file, and pass that list of lines back to the sort routine,
     which will then sort those lines and return them as a list
     to whatever the context of the sort was.

     Assignment is a little bit special in that it uses its left
     argument to determine the context for the right argument.
     Assignment to a scalar evaluates the right-hand side in
     scalar context, while assignment to an array or hash
     evaluates the righthand side in list context.  Assignment to
     a list (or slice, which is just a list anyway) also
     evaluates the righthand side in list context.

     When you use the "use warnings" pragma or Perl's -w command-
     line option, you may see warnings about useless uses of
     constants or functions in "void context".  Void context just
     means the value has been discarded, such as a statement
     containing only ""fred";" or "getpwuid(0);".  It still
     counts as scalar context for functions that care whether or
     not they're being called in list context.

     User-defined subroutines may choose to care whether they are
     being called in a void, scalar, or list context.  Most
     subroutines do not need to bother, though.  That's because
     both scalars and lists are automatically interpolated into

perl v5.12.5         Last change: 2012-11-03                    3

Perl Programmers Reference Guide                      PERLDATA(1)

     lists.  See "wantarray" in perlfunc for how you would
     dynamically discern your function's calling context.

  Scalar values
     All data in Perl is a scalar, an array of scalars, or a hash
     of scalars.  A scalar may contain one single value in any of
     three different flavors: a number, a string, or a reference.
     In general, conversion from one form to another is
     transparent.  Although a scalar may not directly hold
     multiple values, it may contain a reference to an array or
     hash which in turn contains multiple values.

     Scalars aren't necessarily one thing or another.  There's no
     place to declare a scalar variable to be of type "string",
     type "number", type "reference", or anything else.  Because
     of the automatic conversion of scalars, operations that
     return scalars don't need to care (and in fact, cannot care)
     whether their caller is looking for a string, a number, or a
     reference.  Perl is a contextually polymorphic language
     whose scalars can be strings, numbers, or references (which
     includes objects).  Although strings and numbers are
     considered pretty much the same thing for nearly all
     purposes, references are strongly-typed, uncastable pointers
     with builtin reference-counting and destructor invocation.

     A scalar value is interpreted as TRUE in the Boolean sense
     if it is not the null string or the number 0 (or its string
     equivalent, "0").  The Boolean context is just a special
     kind of scalar context where no conversion to a string or a
     number is ever performed.

     There are actually two varieties of null strings (sometimes
     referred to as "empty" strings), a defined one and an
     undefined one.  The defined version is just a string of
     length zero, such as "".  The undefined version is the value
     that indicates that there is no real value for something,
     such as when there was an error, or at end of file, or when
     you refer to an uninitialized variable or element of an
     array or hash.  Although in early versions of Perl, an
     undefined scalar could become defined when first used in a
     place expecting a defined value, this no longer happens
     except for rare cases of autovivification as explained in
     perlref.  You can use the defined() operator to determine
     whether a scalar value is defined (this has no meaning on
     arrays or hashes), and the undef() operator to produce an
     undefined value.

     To find out whether a given string is a valid non-zero
     number, it's sometimes enough to test it against both
     numeric 0 and also lexical "0" (although this will cause
     noises if warnings are on).  That's because strings that
     aren't numbers count as 0, just as they do in awk:

perl v5.12.5         Last change: 2012-11-03                    4

Perl Programmers Reference Guide                      PERLDATA(1)

         if ($str == 0 && $str ne "0")  {
             warn "That doesn't look like a number";

     That method may be best because otherwise you won't treat
     IEEE notations like "NaN" or "Infinity" properly.  At other
     times, you might prefer to determine whether string data can
     be used numerically by calling the POSIX::strtod() function
     or by inspecting your string with a regular expression (as
     documented in perlre).

         warn "has nondigits"        if     /\D/;
         warn "not a natural number" unless /^\d+$/;             # rejects -3
         warn "not an integer"       unless /^-?\d+$/;           # rejects +3
         warn "not an integer"       unless /^[+-]?\d+$/;
         warn "not a decimal number" unless /^-?\d+\.?\d*$/;     # rejects .2
         warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
         warn "not a C float"
             unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

     The length of an array is a scalar value.  You may find the
     length of array @days by evaluating $#days, as in csh.
     However, this isn't the length of the array; it's the
     subscript of the last element, which is a different value
     since there is ordinarily a 0th element.  Assigning to
     $#days actually changes the length of the array.  Shortening
     an array this way destroys intervening values.  Lengthening
     an array that was previously shortened does not recover
     values that were in those elements.  (It used to do so in
     Perl 4, but we had to break this to make sure destructors
     were called when expected.)

     You can also gain some minuscule measure of efficiency by
     pre-extending an array that is going to get big.  You can
     also extend an array by assigning to an element that is off
     the end of the array.  You can truncate an array down to
     nothing by assigning the null list () to it.  The following
     are equivalent:

         @whatever = ();
         $#whatever = -1;

     If you evaluate an array in scalar context, it returns the
     length of the array.  (Note that this is not true of lists,
     which return the last value, like the C comma operator, nor
     of built-in functions, which return whatever they feel like
     returning.)  The following is always true:

         scalar(@whatever) == $#whatever - $[ + 1;

     Version 5 of Perl changed the semantics of $[: files that
     don't set the value of $[ no longer need to worry about

perl v5.12.5         Last change: 2012-11-03                    5

Perl Programmers Reference Guide                      PERLDATA(1)

     whether another file changed its value.  (In other words,
     use of $[ is deprecated.)  So in general you can assume that

         scalar(@whatever) == $#whatever + 1;

     Some programmers choose to use an explicit conversion so as
     to leave nothing to doubt:

         $element_count = scalar(@whatever);

     If you evaluate a hash in scalar context, it returns false
     if the hash is empty.  If there are any key/value pairs, it
     returns true; more precisely, the value returned is a string
     consisting of the number of used buckets and the number of
     allocated buckets, separated by a slash.  This is pretty
     much useful only to find out whether Perl's internal hashing
     algorithm is performing poorly on your data set.  For
     example, you stick 10,000 things in a hash, but evaluating
     %HASH in scalar context reveals "1/16", which means only one
     out of sixteen buckets has been touched, and presumably
     contains all 10,000 of your items.  This isn't supposed to
     happen.  If a tied hash is evaluated in scalar context, a
     fatal error will result, since this bucket usage information
     is currently not available for tied hashes.

     You can preallocate space for a hash by assigning to the
     keys() function.  This rounds up the allocated buckets to
     the next power of two:

         keys(%users) = 1000;                # allocate 1024 buckets

  Scalar value constructors
     Numeric literals are specified in any of the following
     floating point or integer formats:

         .23E-10             # a very small number
         3.14_15_92          # a very important number
         4_294_967_296       # underscore for legibility
         0xff                # hex
         0xdead_beef         # more hex
         0377                # octal (only numbers, begins with 0)
         0b011011            # binary

     You are allowed to use underscores (underbars) in numeric
     literals between digits for legibility.  You could, for
     example, group binary digits by threes (as for a Unix-style
     mode argument such as 0b110_100_100) or by fours (to
     represent nibbles, as in 0b1010_0110) or in other groups.

perl v5.12.5         Last change: 2012-11-03                    6

Perl Programmers Reference Guide                      PERLDATA(1)

     String literals are usually delimited by either single or
     double quotes.  They work much like quotes in the standard
     Unix shells: double-quoted string literals are subject to
     backslash and variable substitution; single-quoted strings
     are not (except for "\'" and "\\").  The usual C-style
     backslash rules apply for making characters such as newline,
     tab, etc., as well as some more exotic forms.  See "Quote
     and Quote-like Operators" in perlop for a list.

     Hexadecimal, octal, or binary, representations in string
     literals (e.g. '0xff') are not automatically converted to
     their integer representation.  The hex() and oct() functions
     make these conversions for you.  See "hex" in perlfunc and
     "oct" in perlfunc for more details.

     You can also embed newlines directly in your strings, i.e.,
     they can end on a different line than they begin.  This is
     nice, but if you forget your trailing quote, the error will
     not be reported until Perl finds another line containing the
     quote character, which may be much further on in the script.
     Variable substitution inside strings is limited to scalar
     variables, arrays, and array or hash slices.  (In other
     words, names beginning with $ or @, followed by an optional
     bracketed expression as a subscript.)  The following code
     segment prints out "The price is $100."

         $Price = '$100';    # not interpolated
         print "The price is $Price.\n";     # interpolated

     There is no double interpolation in Perl, so the $100 is
     left as is.

     By default floating point numbers substituted inside strings
     use the dot (".")  as the decimal separator.  If "use
     locale" is in effect, and POSIX::setlocale() has been
     called, the character used for the decimal separator is
     affected by the LC_NUMERIC locale.  See perllocale and

     As in some shells, you can enclose the variable name in
     braces to disambiguate it from following alphanumerics (and
     underscores).  You must also do this when interpolating a
     variable into a string to separate the variable name from a
     following double-colon or an apostrophe, since these would
     be otherwise treated as a package separator:

         $who = "Larry";
         print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
         print "We use ${who}speak when ${who}'s here.\n";

     Without the braces, Perl would have looked for a $whospeak,
     a $who::0, and a "$who's" variable.  The last two would be

perl v5.12.5         Last change: 2012-11-03                    7

Perl Programmers Reference Guide                      PERLDATA(1)

     the $0 and the $s variables in the (presumably) non-existent
     package "who".

     In fact, an identifier within such curlies is forced to be a
     string, as is any simple identifier within a hash subscript.
     Neither need quoting.  Our earlier example, $days{'Feb'} can
     be written as $days{Feb} and the quotes will be assumed
     automatically.  But anything more complicated in the
     subscript will be interpreted as an expression.  This means
     for example that "$version{2.0}++" is equivalent to
     "$version{2}++", not to "$version{'2.0'}++".

     Version Strings

     A literal of the form "v1.20.300.4000" is parsed as a string
     composed of characters with the specified ordinals.  This
     form, known as v-strings, provides an alternative, more
     readable way to construct strings, rather than use the
     somewhat less readable interpolation form
     "\x{1}\x{14}\x{12c}\x{fa0}".  This is useful for
     representing Unicode strings, and for comparing version
     "numbers" using the string comparison operators, "cmp",
     "gt", "lt" etc.  If there are two or more dots in the
     literal, the leading "v" may be omitted.

         print v9786;              # prints SMILEY, "\x{263a}"
         print v102.111.111;       # prints "foo"
         print 102.111.111;        # same

     Such literals are accepted by both "require" and "use" for
     doing a version check.  Note that using the v-strings for
     IPv4 addresses is not portable unless you also use the
     inet_aton()/inet_ntoa() routines of the Socket package.

     Note that since Perl 5.8.1 the single-number v-strings (like
     "v65") are not v-strings before the "=>" operator (which is
     usually used to separate a hash key from a hash value),
     instead they are interpreted as literal strings ('v65').
     They were v-strings from Perl 5.6.0 to Perl 5.8.0, but that
     caused more confusion and breakage than good.  Multi-number
     v-strings like "v65.66" and 65.66.67 continue to be
     v-strings always.

     Special Literals

     The special literals __FILE__, __LINE__, and __PACKAGE__
     represent the current filename, line number, and package
     name at that point in your program.  They may be used only
     as separate tokens; they will not be interpolated into
     strings.  If there is no current package (due to an empty
     "package;" directive), __PACKAGE__ is the undefined value.

perl v5.12.5         Last change: 2012-11-03                    8

Perl Programmers Reference Guide                      PERLDATA(1)

     The two control characters ^D and ^Z, and the tokens __END__
     and __DATA__ may be used to indicate the logical end of the
     script before the actual end of file.  Any following text is

     Text after __DATA__ may be read via the filehandle
     "PACKNAME::DATA", where "PACKNAME" is the package that was
     current when the __DATA__ token was encountered.  The
     filehandle is left open pointing to the contents after
     __DATA__.  It is the program's responsibility to "close
     DATA" when it is done reading from it.  For compatibility
     with older scripts written before __DATA__ was introduced,
     __END__ behaves like __DATA__ in the top level script (but
     not in files loaded with "require" or "do") and leaves the
     remaining contents of the file accessible via "main::DATA".

     See SelfLoader for more description of __DATA__, and an
     example of its use.  Note that you cannot read from the DATA
     filehandle in a BEGIN block: the BEGIN block is executed as
     soon as it is seen (during compilation), at which point the
     corresponding __DATA__ (or __END__) token has not yet been


     A word that has no other interpretation in the grammar will
     be treated as if it were a quoted string.  These are known
     as "barewords".  As with filehandles and labels, a bareword
     that consists entirely of lowercase letters risks conflict
     with future reserved words, and if you use the "use
     warnings" pragma or the -w switch, Perl will warn you about
     any such words.  Perl limits barewords (like identifiers) to
     about 250 characters.  Future versions of Perl are likely to
     eliminate these arbitrary limitations.

     Some people may wish to outlaw barewords entirely.  If you

         use strict 'subs';

     then any bareword that would NOT be interpreted as a
     subroutine call produces a compile-time error instead.  The
     restriction lasts to the end of the enclosing block.  An
     inner block may countermand this by saying "no strict

     Array Joining Delimiter

     Arrays and slices are interpolated into double-quoted
     strings by joining the elements with the delimiter specified
     in the $" variable ($LIST_SEPARATOR if "use English;" is
     specified), space by default.  The following are equivalent:

perl v5.12.5         Last change: 2012-11-03                    9

Perl Programmers Reference Guide                      PERLDATA(1)

         $temp = join($", @ARGV);
         system "echo $temp";

         system "echo @ARGV";

     Within search patterns (which also undergo double-quotish
     substitution) there is an unfortunate ambiguity:  Is
     "/$foo[bar]/" to be interpreted as "/${foo}[bar]/" (where
     "[bar]" is a character class for the regular expression) or
     as "/${foo[bar]}/" (where "[bar]" is the subscript to array
     @foo)?  If @foo doesn't otherwise exist, then it's obviously
     a character class.  If @foo exists, Perl takes a good guess
     about "[bar]", and is almost always right.  If it does guess
     wrong, or if you're just plain paranoid, you can force the
     correct interpretation with curly braces as above.

     If you're looking for the information on how to use here-
     documents, which used to be here, that's been moved to
     "Quote and Quote-like Operators" in perlop.

  List value constructors
     List values are denoted by separating individual values by
     commas (and enclosing the list in parentheses where
     precedence requires it):


     In a context not requiring a list value, the value of what
     appears to be a list literal is simply the value of the
     final element, as with the C comma operator.  For example,

         @foo = ('cc', '-E', $bar);

     assigns the entire list value to array @foo, but

         $foo = ('cc', '-E', $bar);

     assigns the value of variable $bar to the scalar variable
     $foo.  Note that the value of an actual array in scalar
     context is the length of the array; the following assigns
     the value 3 to $foo:

         @foo = ('cc', '-E', $bar);
         $foo = @foo;                # $foo gets 3

     You may have an optional comma before the closing
     parenthesis of a list literal, so that you can say:

perl v5.12.5         Last change: 2012-11-03                   10

Perl Programmers Reference Guide                      PERLDATA(1)

         @foo = (

     To use a here-document to assign an array, one line per
     element, you might use an approach like this:

         @sauces = <<End_Lines =~ m/(\S.*\S)/g;
             normal tomato
             spicy tomato
             green chile
             white wine

     LISTs do automatic interpolation of sublists.  That is, when
     a LIST is evaluated, each element of the list is evaluated
     in list context, and the resulting list value is
     interpolated into LIST just as if each individual element
     were a member of LIST.  Thus arrays and hashes lose their
     identity in a LIST--the list


     contains all the elements of @foo followed by all the
     elements of @bar, followed by all the elements returned by
     the subroutine named SomeSub called in list context,
     followed by the key/value pairs of %glarch.  To make a list
     reference that does NOT interpolate, see perlref.

     The null list is represented by ().  Interpolating it in a
     list has no effect.  Thus ((),(),()) is equivalent to ().
     Similarly, interpolating an array with no elements is the
     same as if no array had been interpolated at that point.

     This interpolation combines with the facts that the opening
     and closing parentheses are optional (except when necessary
     for precedence) and lists may end with an optional comma to
     mean that multiple commas within lists are legal syntax. The
     list "1,,3" is a concatenation of two lists, "1," and 3, the
     first of which ends with that optional comma.  "1,,3" is
     "(1,),(3)" is "1,3" (And similarly for "1,,,3" is
     "(1,),(,),3" is "1,3" and so on.)  Not that we'd advise you
     to use this obfuscation.

     A list value may also be subscripted like a normal array.
     You must put the list in parentheses to avoid ambiguity.
     For example:

perl v5.12.5         Last change: 2012-11-03                   11

Perl Programmers Reference Guide                      PERLDATA(1)

         # Stat returns list value.
         $time = (stat($file))[8];

         $time = stat($file)[8];  # OOPS, FORGOT PARENTHESES

         # Find a hex digit.
         $hexdigit = ('a','b','c','d','e','f')[$digit-10];

         # A "reverse comma operator".
         return (pop(@foo),pop(@foo))[0];

     Lists may be assigned to only when each element of the list
     is itself legal to assign to:

         ($a, $b, $c) = (1, 2, 3);

         ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);

     An exception to this is that you may assign to "undef" in a
     list.  This is useful for throwing away some of the return
     values of a function:

         ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

     List assignment in scalar context returns the number of
     elements produced by the expression on the right side of the

         $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
         $x = (($foo,$bar) = f());           # set $x to f()'s return count

     This is handy when you want to do a list assignment in a
     Boolean context, because most list functions return a null
     list when finished, which when assigned produces a 0, which
     is interpreted as FALSE.

     It's also the source of a useful idiom for executing a
     function or performing an operation in list context and then
     counting the number of return values, by assigning to an
     empty list and then using that assignment in scalar context.
     For example, this code:

         $count = () = $string =~ /\d+/g;

     will place into $count the number of digit groups found in
     $string.  This happens because the pattern match is in list
     context (since it is being assigned to the empty list), and
     will therefore return a list of all matching parts of the
     string. The list assignment in scalar context will translate
     that into the number of elements (here, the number of times
     the pattern matched) and assign that to $count. Note that

perl v5.12.5         Last change: 2012-11-03                   12

Perl Programmers Reference Guide                      PERLDATA(1)

     simply using

         $count = $string =~ /\d+/g;

     would not have worked, since a pattern match in scalar
     context will only return true or false, rather than a count
     of matches.

     The final element of a list assignment may be an array or a

         ($a, $b, @rest) = split;
         my($a, $b, %rest) = @_;

     You can actually put an array or hash anywhere in the list,
     but the first one in the list will soak up all the values,
     and anything after it will become undefined.  This may be
     useful in a my() or local().

     A hash can be initialized using a literal list holding pairs
     of items to be interpreted as a key and a value:

         # same as map assignment above
         %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);

     While literal lists and named arrays are often
     interchangeable, that's not the case for hashes.  Just
     because you can subscript a list value like a normal array
     does not mean that you can subscript a list value as a hash.
     Likewise, hashes included as parts of other lists (including
     parameters lists and return lists from functions) always
     flatten out into key/value pairs.  That's why it's good to
     use references sometimes.

     It is often more readable to use the "=>" operator between
     key/value pairs.  The "=>" operator is mostly just a more
     visually distinctive synonym for a comma, but it also
     arranges for its left-hand operand to be interpreted as a
     string if it's a bareword that would be a legal simple
     identifier. "=>" doesn't quote compound identifiers, that
     contain double colons. This makes it nice for initializing

         %map = (
                      red   => 0x00f,
                      blue  => 0x0f0,
                      green => 0xf00,

     or for initializing hash references to be used as records:

perl v5.12.5         Last change: 2012-11-03                   13

Perl Programmers Reference Guide                      PERLDATA(1)

         $rec = {
                     witch => 'Mable the Merciless',
                     cat   => 'Fluffy the Ferocious',
                     date  => '10/31/1776',

     or for using call-by-named-parameter to complicated

        $field = $query->radio_group(
                    name      => 'group_name',
                    values    => ['eenie','meenie','minie'],
                    default   => 'meenie',
                    linebreak => 'true',
                    labels    => \%labels

     Note that just because a hash is initialized in that order
     doesn't mean that it comes out in that order.  See "sort" in
     perlfunc for examples of how to arrange for an output

     An array is subscripted by specifying a dollar sign ("$"),
     then the name of the array (without the leading "@"), then
     the subscript inside square brackets.  For example:

         @myarray = (5, 50, 500, 5000);
         print "The Third Element is", $myarray[2], "\n";

     The array indices start with 0. A negative subscript
     retrieves its value from the end.  In our example,
     $myarray[-1] would have been 5000, and $myarray[-2] would
     have been 500.

     Hash subscripts are similar, only instead of square brackets
     curly brackets are used. For example:

         %scientists =
             "Newton" => "Isaac",
             "Einstein" => "Albert",
             "Darwin" => "Charles",
             "Feynman" => "Richard",

         print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";

     A common way to access an array or a hash is one scalar
     element at a time.  You can also subscript a list to get a
     single element from it.

perl v5.12.5         Last change: 2012-11-03                   14

Perl Programmers Reference Guide                      PERLDATA(1)

         $whoami = $ENV{"USER"};             # one element from the hash
         $parent = $ISA[0];                  # one element from the array
         $dir    = (getpwnam("daemon"))[7];  # likewise, but with list

     A slice accesses several elements of a list, an array, or a
     hash simultaneously using a list of subscripts.  It's more
     convenient than writing out the individual elements as a
     list of separate scalar values.

         ($him, $her)   = @folks[0,-1];              # array slice
         @them          = @folks[0 .. 3];            # array slice
         ($who, $home)  = @ENV{"USER", "HOME"};      # hash slice
         ($uid, $dir)   = (getpwnam("daemon"))[2,7]; # list slice

     Since you can assign to a list of variables, you can also
     assign to an array or hash slice.

         @days[3..5]    = qw/Wed Thu Fri/;
                        = (0xff0000, 0x0000ff, 0x00ff00);
         @folks[0, -1]  = @folks[-1, 0];

     The previous assignments are exactly equivalent to

         ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
         ($colors{'red'}, $colors{'blue'}, $colors{'green'})
                        = (0xff0000, 0x0000ff, 0x00ff00);
         ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);

     Since changing a slice changes the original array or hash
     that it's slicing, a "foreach" construct will alter some--or
     even all--of the values of the array or hash.

         foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }

         foreach (@hash{qw[key1 key2]}) {
             s/^\s+//;           # trim leading whitespace
             s/\s+$//;           # trim trailing whitespace
             s/(\w+)/\u\L$1/g;   # "titlecase" words

     A slice of an empty list is still an empty list.  Thus:

         @a = ()[1,0];           # @a has no elements
         @b = (@a)[0,1];         # @b has no elements
         @c = (0,1)[2,3];        # @c has no elements


         @a = (1)[1,0];          # @a has two elements
         @b = (1,undef)[1,0,2];  # @b has three elements

perl v5.12.5         Last change: 2012-11-03                   15

Perl Programmers Reference Guide                      PERLDATA(1)

     This makes it easy to write loops that terminate when a null
     list is returned:

         while ( ($home, $user) = (getpwent)[7,0]) {
             printf "%-8s %s\n", $user, $home;

     As noted earlier in this document, the scalar sense of list
     assignment is the number of elements on the right-hand side
     of the assignment.  The null list contains no elements, so
     when the password file is exhausted, the result is 0, not 2.

     If you're confused about why you use an '@' there on a hash
     slice instead of a '%', think of it like this.  The type of
     bracket (square or curly) governs whether it's an array or a
     hash being looked at.  On the other hand, the leading symbol
     ('$' or '@') on the array or hash indicates whether you are
     getting back a singular value (a scalar) or a plural one (a

  Typeglobs and Filehandles
     Perl uses an internal type called a typeglob to hold an
     entire symbol table entry.  The type prefix of a typeglob is
     a "*", because it represents all types.  This used to be the
     preferred way to pass arrays and hashes by reference into a
     function, but now that we have real references, this is
     seldom needed.

     The main use of typeglobs in modern Perl is create symbol
     table aliases.  This assignment:

         *this = *that;

     makes $this an alias for $that, @this an alias for @that,
     %this an alias for %that, &this an alias for &that, etc.
     Much safer is to use a reference.  This:

         local *Here::blue = \$There::green;

     temporarily makes $Here::blue an alias for $There::green,
     but doesn't make @Here::blue an alias for @There::green, or
     %Here::blue an alias for %There::green, etc.  See "Symbol
     Tables" in perlmod for more examples of this.  Strange
     though this may seem, this is the basis for the whole module
     import/export system.

     Another use for typeglobs is to pass filehandles into a
     function or to create new filehandles.  If you need to use a
     typeglob to save away a filehandle, do it this way:

         $fh = *STDOUT;

perl v5.12.5         Last change: 2012-11-03                   16

Perl Programmers Reference Guide                      PERLDATA(1)

     or perhaps as a real reference, like this:

         $fh = \*STDOUT;

     See perlsub for examples of using these as indirect
     filehandles in functions.

     Typeglobs are also a way to create a local filehandle using
     the local() operator.  These last until their block is
     exited, but may be passed back.  For example:

         sub newopen {
             my $path = shift;
             local  *FH;  # not my!
             open   (FH, $path)          or  return undef;
             return *FH;
         $fh = newopen('/etc/passwd');

     Now that we have the *foo{THING} notation, typeglobs aren't
     used as much for filehandle manipulations, although they're
     still needed to pass brand new file and directory handles
     into or out of functions. That's because *HANDLE{IO} only
     works if HANDLE has already been used as a handle.  In other
     words, *FH must be used to create new symbol table entries;
     *foo{THING} cannot.  When in doubt, use *FH.

     All functions that are capable of creating filehandles
     (open(), opendir(), pipe(), socketpair(), sysopen(),
     socket(), and accept()) automatically create an anonymous
     filehandle if the handle passed to them is an uninitialized
     scalar variable. This allows the constructs such as "open(my
     $fh, ...)" and "open(local $fh,...)" to be used to create
     filehandles that will conveniently be closed automatically
     when the scope ends, provided there are no other references
     to them. This largely eliminates the need for typeglobs when
     opening filehandles that must be passed around, as in the
     following example:

         sub myopen {
             open my $fh, "@_"
                  or die "Can't open '@_': $!";
             return $fh;

             my $f = myopen("</etc/motd");
             print <$f>;
             # $f implicitly closed here

perl v5.12.5         Last change: 2012-11-03                   17

Perl Programmers Reference Guide                      PERLDATA(1)

     Note that if an initialized scalar variable is used instead
     the result is different: "my $fh='zzz'; open($fh, ...)" is
     equivalent to "open( *{'zzz'}, ...)".  "use strict 'refs'"
     forbids such practice.

     Another way to create anonymous filehandles is with the
     Symbol module or with the IO::Handle module and its ilk.
     These modules have the advantage of not hiding different
     types of the same name during the local().  See the bottom
     of "open()" in perlfunc for an example.

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     See perlvar for a description of Perl's built-in variables
     and a discussion of legal variable names.  See perlref,
     perlsub, and "Symbol Tables" in perlmod for more discussion
     on typeglobs and the *foo{THING} syntax.

     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

perl v5.12.5         Last change: 2012-11-03                   18