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perlsyn (1)


perlsyn - Perl syntax


Please see following description for synopsis


Perl Programmers Reference Guide                       PERLSYN(1)

     perlsyn - Perl syntax

     A Perl program consists of a sequence of declarations and
     statements which run from the top to the bottom.  Loops,
     subroutines and other control structures allow you to jump
     around within the code.

     Perl is a free-form language, you can format and indent it
     however you like.  Whitespace mostly serves to separate
     tokens, unlike languages like Python where it is an
     important part of the syntax.

     Many of Perl's syntactic elements are optional.  Rather than
     requiring you to put parentheses around every function call
     and declare every variable, you can often leave such
     explicit elements off and Perl will figure out what you
     meant.  This is known as Do What I Mean, abbreviated DWIM.
     It allows programmers to be lazy and to code in a style with
     which they are comfortable.

     Perl borrows syntax and concepts from many languages: awk,
     sed, C, Bourne Shell, Smalltalk, Lisp and even English.
     Other languages have borrowed syntax from Perl, particularly
     its regular expression extensions.  So if you have
     programmed in another language you will see familiar pieces
     in Perl.  They often work the same, but see perltrap for
     information about how they differ.

     The only things you need to declare in Perl are report
     formats and subroutines (and sometimes not even
     subroutines).  A variable holds the undefined value
     ("undef") until it has been assigned a defined value, which
     is anything other than "undef".  When used as a number,
     "undef" is treated as 0; when used as a string, it is
     treated as the empty string, ""; and when used as a
     reference that isn't being assigned to, it is treated as an
     error.  If you enable warnings, you'll be notified of an
     uninitialized value whenever you treat "undef" as a string
     or a number.  Well, usually.  Boolean contexts, such as:

         my $a;
         if ($a) {}

     are exempt from warnings (because they care about truth
     rather than definedness).  Operators such as "++", "--",
     "+=", "-=", and ".=", that operate on undefined left values
     such as:

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         my $a;

     are also always exempt from such warnings.

     A declaration can be put anywhere a statement can, but has
     no effect on the execution of the primary sequence of
     statements--declarations all take effect at compile time.
     Typically all the declarations are put at the beginning or
     the end of the script.  However, if you're using lexically-
     scoped private variables created with "my()", you'll have to
     make sure your format or subroutine definition is within the
     same block scope as the my if you expect to be able to
     access those private variables.

     Declaring a subroutine allows a subroutine name to be used
     as if it were a list operator from that point forward in the
     program.  You can declare a subroutine without defining it
     by saying "sub name", thus:

         sub myname;
         $me = myname $0             or die "can't get myname";

     Note that myname() functions as a list operator, not as a
     unary operator; so be careful to use "or" instead of "||" in
     this case.  However, if you were to declare the subroutine
     as "sub myname ($)", then "myname" would function as a unary
     operator, so either "or" or "||" would work.

     Subroutines declarations can also be loaded up with the
     "require" statement or both loaded and imported into your
     namespace with a "use" statement.  See perlmod for details
     on this.

     A statement sequence may contain declarations of lexically-
     scoped variables, but apart from declaring a variable name,
     the declaration acts like an ordinary statement, and is
     elaborated within the sequence of statements as if it were
     an ordinary statement.  That means it actually has both
     compile-time and run-time effects.

     Text from a "#" character until the end of the line is a
     comment, and is ignored.  Exceptions include "#" inside a
     string or regular expression.

  Simple Statements
     The only kind of simple statement is an expression evaluated
     for its side effects.  Every simple statement must be
     terminated with a semicolon, unless it is the final
     statement in a block, in which case the semicolon is
     optional.  (A semicolon is still encouraged if the block

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     takes up more than one line, because you may eventually add
     another line.)  Note that there are some operators like
     "eval {}" and "do {}" that look like compound statements,
     but aren't (they're just TERMs in an expression), and thus
     need an explicit termination if used as the last item in a

  Truth and Falsehood
     The number 0, the strings '0' and '', the empty list "()",
     and "undef" are all false in a boolean context. All other
     values are true.  Negation of a true value by "!" or "not"
     returns a special false value.  When evaluated as a string
     it is treated as '', but as a number, it is treated as 0.

  Statement Modifiers
     Any simple statement may optionally be followed by a SINGLE
     modifier, just before the terminating semicolon (or block
     ending).  The possible modifiers are:

         if EXPR
         unless EXPR
         while EXPR
         until EXPR
         when EXPR
         for LIST
         foreach LIST

     The "EXPR" following the modifier is referred to as the
     "condition".  Its truth or falsehood determines how the
     modifier will behave.

     "if" executes the statement once if and only if the
     condition is true.  "unless" is the opposite, it executes
     the statement unless the condition is true (i.e., if the
     condition is false).

         print "Basset hounds got long ears" if length $ear >= 10;
         go_outside() and play() unless $is_raining;

     "when" executes the statement when $_ smart matches "EXPR",
     and then either "break"s out if it's enclosed in a "given"
     scope or skips to the "next" element when it lies directly
     inside a "for" loop.  See also "Switch statements".

         given ($something) {
             $abc    = 1 when /^abc/;
             $just_a = 1 when /^a/;
             $other  = 1;

         for (@names) {
             admin($_)   when [ qw/Alice Bob/ ];

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             regular($_) when [ qw/Chris David Ellen/ ];

     The "foreach" modifier is an iterator: it executes the
     statement once for each item in the LIST (with $_ aliased to
     each item in turn).

         print "Hello $_!\n" foreach qw(world Dolly nurse);

     "while" repeats the statement while the condition is true.
     "until" does the opposite, it repeats the statement until
     the condition is true (or while the condition is false):

         # Both of these count from 0 to 10.
         print $i++ while $i <= 10;
         print $j++ until $j >  10;

     The "while" and "until" modifiers have the usual ""while"
     loop" semantics (conditional evaluated first), except when
     applied to a "do"-BLOCK (or to the deprecated
     "do"-SUBROUTINE statement), in which case the block executes
     once before the conditional is evaluated.  This is so that
     you can write loops like:

         do {
             $line = <STDIN>;
         } until $line  eq ".\n";

     See "do" in perlfunc.  Note also that the loop control
     statements described later will NOT work in this construct,
     because modifiers don't take loop labels.  Sorry.  You can
     always put another block inside of it (for "next") or around
     it (for "last") to do that sort of thing.  For "next", just
     double the braces:

         do {{
             next if $x == $y;
             # do something here
         }} until $x++ > $z;

     For "last", you have to be more elaborate:

         LOOP: {
                 do {
                     last if $x = $y**2;
                     # do something here
                 } while $x++ <= $z;

     NOTE: The behaviour of a "my" statement modified with a
     statement modifier conditional or loop construct (e.g. "my

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     $x if ...") is undefined.  The value of the "my" variable
     may be "undef", any previously assigned value, or possibly
     anything else.  Don't rely on it.  Future versions of perl
     might do something different from the version of perl you
     try it out on.  Here be dragons.

  Compound Statements
     In Perl, a sequence of statements that defines a scope is
     called a block.  Sometimes a block is delimited by the file
     containing it (in the case of a required file, or the
     program as a whole), and sometimes a block is delimited by
     the extent of a string (in the case of an eval).

     But generally, a block is delimited by curly brackets, also
     known as braces.  We will call this syntactic construct a

     The following compound statements may be used to control

         if (EXPR) BLOCK
         if (EXPR) BLOCK else BLOCK
         if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
         unless (EXPR) BLOCK
         unless (EXPR) BLOCK else BLOCK
         unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
         LABEL while (EXPR) BLOCK
         LABEL while (EXPR) BLOCK continue BLOCK
         LABEL until (EXPR) BLOCK
         LABEL until (EXPR) BLOCK continue BLOCK
         LABEL foreach VAR (LIST) BLOCK
         LABEL foreach VAR (LIST) BLOCK continue BLOCK
         LABEL BLOCK continue BLOCK

     Note that, unlike C and Pascal, these are defined in terms
     of BLOCKs, not statements.  This means that the curly
     brackets are required--no dangling statements allowed.  If
     you want to write conditionals without curly brackets there
     are several other ways to do it.  The following all do the
     same thing:

         if (!open(FOO)) { die "Can't open $FOO: $!"; }
         die "Can't open $FOO: $!" unless open(FOO);
         open(FOO) or die "Can't open $FOO: $!";     # FOO or bust!
         open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
                             # a bit exotic, that last one

     The "if" statement is straightforward.  Because BLOCKs are
     always bounded by curly brackets, there is never any
     ambiguity about which "if" an "else" goes with.  If you use
     "unless" in place of "if", the sense of the test is

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     reversed. Like "if", "unless" can be followed by "else".
     "unless" can even be followed by one or more "elsif"
     statements, though you may want to think twice before using
     that particular language construct, as everyone reading your
     code will have to think at least twice before they can
     understand what's going on.

     The "while" statement executes the block as long as the
     expression is true.  The "until" statement executes the
     block as long as the expression is false.  The LABEL is
     optional, and if present, consists of an identifier followed
     by a colon.  The LABEL identifies the loop for the loop
     control statements "next", "last", and "redo".  If the LABEL
     is omitted, the loop control statement refers to the
     innermost enclosing loop.  This may include dynamically
     looking back your call-stack at run time to find the LABEL.
     Such desperate behavior triggers a warning if you use the
     "use warnings" pragma or the -w flag.

     If there is a "continue" BLOCK, it is always executed just
     before the conditional is about to be evaluated again.  Thus
     it can be used to increment a loop variable, even when the
     loop has been continued via the "next" statement.

     Extension modules can also hook into the Perl parser to
     define new kinds of compound statement.  These are
     introduced by a keyword which the extension recognises, and
     the syntax following the keyword is defined entirely by the
     extension.  If you are an implementor, see
     "PL_keyword_plugin" in perlapi for the mechanism.  If you
     are using such a module, see the module's documentation for
     details of the syntax that it defines.

  Loop Control
     The "next" command starts the next iteration of the loop:

         LINE: while (<STDIN>) {
             next LINE if /^#/;      # discard comments

     The "last" command immediately exits the loop in question.
     The "continue" block, if any, is not executed:

         LINE: while (<STDIN>) {
             last LINE if /^$/;      # exit when done with header

     The "redo" command restarts the loop block without
     evaluating the conditional again.  The "continue" block, if
     any, is not executed.  This command is normally used by

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     programs that want to lie to themselves about what was just

     For example, when processing a file like /etc/termcap.  If
     your input lines might end in backslashes to indicate
     continuation, you want to skip ahead and get the next

         while (<>) {
             if (s/\\$//) {
                 $_ .= <>;
                 redo unless eof();
             # now process $_

     which is Perl short-hand for the more explicitly written

         LINE: while (defined($line = <ARGV>)) {
             if ($line =~ s/\\$//) {
                 $line .= <ARGV>;
                 redo LINE unless eof(); # not eof(ARGV)!
             # now process $line

     Note that if there were a "continue" block on the above
     code, it would get executed only on lines discarded by the
     regex (since redo skips the continue block). A continue
     block is often used to reset line counters or "?pat?" one-
     time matches:

         # inspired by :1,$g/fred/s//WILMA/
         while (<>) {
             ?(fred)?    && s//WILMA $1 WILMA/;
             ?(barney)?  && s//BETTY $1 BETTY/;
             ?(homer)?   && s//MARGE $1 MARGE/;
         } continue {
             print "$ARGV $.: $_";
             close ARGV  if eof();           # reset $.
             reset       if eof();           # reset ?pat?

     If the word "while" is replaced by the word "until", the
     sense of the test is reversed, but the conditional is still
     tested before the first iteration.

     The loop control statements don't work in an "if" or
     "unless", since they aren't loops.  You can double the

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     braces to make them such, though.

         if (/pattern/) {{
             last if /fred/;
             next if /barney/; # same effect as "last", but doesn't document as well
             # do something here

     This is caused by the fact that a block by itself acts as a
     loop that executes once, see "Basic BLOCKs".

     The form "while/if BLOCK BLOCK", available in Perl 4, is no
     longer available.   Replace any occurrence of "if BLOCK" by
     "if (do BLOCK)".

  For Loops
     Perl's C-style "for" loop works like the corresponding
     "while" loop; that means that this:

         for ($i = 1; $i < 10; $i++) {

     is the same as this:

         $i = 1;
         while ($i < 10) {
         } continue {

     There is one minor difference: if variables are declared
     with "my" in the initialization section of the "for", the
     lexical scope of those variables is exactly the "for" loop
     (the body of the loop and the control sections).

     Besides the normal array index looping, "for" can lend
     itself to many other interesting applications.  Here's one
     that avoids the problem you get into if you explicitly test
     for end-of-file on an interactive file descriptor causing
     your program to appear to hang.

         $on_a_tty = -t STDIN && -t STDOUT;
         sub prompt { print "yes? " if $on_a_tty }
         for ( prompt(); <STDIN>; prompt() ) {
             # do something

     Using "readline" (or the operator form, "<EXPR>") as the
     conditional of a "for" loop is shorthand for the following.
     This behaviour is the same as a "while" loop conditional.

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         for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
             # do something

  Foreach Loops
     The "foreach" loop iterates over a normal list value and
     sets the variable VAR to be each element of the list in
     turn.  If the variable is preceded with the keyword "my",
     then it is lexically scoped, and is therefore visible only
     within the loop.  Otherwise, the variable is implicitly
     local to the loop and regains its former value upon exiting
     the loop.  If the variable was previously declared with
     "my", it uses that variable instead of the global one, but
     it's still localized to the loop.  This implicit
     localisation occurs only in a "foreach" loop.

     The "foreach" keyword is actually a synonym for the "for"
     keyword, so you can use "foreach" for readability or "for"
     for brevity.  (Or because the Bourne shell is more familiar
     to you than csh, so writing "for" comes more naturally.)  If
     VAR is omitted, $_ is set to each value.

     If any element of LIST is an lvalue, you can modify it by
     modifying VAR inside the loop.  Conversely, if any element
     of LIST is NOT an lvalue, any attempt to modify that element
     will fail.  In other words, the "foreach" loop index
     variable is an implicit alias for each item in the list that
     you're looping over.

     If any part of LIST is an array, "foreach" will get very
     confused if you add or remove elements within the loop body,
     for example with "splice".   So don't do that.

     "foreach" probably won't do what you expect if VAR is a tied
     or other special variable.   Don't do that either.


         for (@ary) { s/foo/bar/ }

         for my $elem (@elements) {
             $elem *= 2;

         for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
             print $count, "\n"; sleep(1);

         for (1..15) { print "Merry Christmas\n"; }

         foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
             print "Item: $item\n";

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     Here's how a C programmer might code up a particular
     algorithm in Perl:

         for (my $i = 0; $i < @ary1; $i++) {
             for (my $j = 0; $j < @ary2; $j++) {
                 if ($ary1[$i] > $ary2[$j]) {
                     last; # can't go to outer :-(
                 $ary1[$i] += $ary2[$j];
             # this is where that last takes me

     Whereas here's how a Perl programmer more comfortable with
     the idiom might do it:

         OUTER: for my $wid (@ary1) {
         INNER:   for my $jet (@ary2) {
                     next OUTER if $wid > $jet;
                     $wid += $jet;

     See how much easier this is?  It's cleaner, safer, and
     faster.  It's cleaner because it's less noisy.  It's safer
     because if code gets added between the inner and outer loops
     later on, the new code won't be accidentally executed.  The
     "next" explicitly iterates the other loop rather than merely
     terminating the inner one.  And it's faster because Perl
     executes a "foreach" statement more rapidly than it would
     the equivalent "for" loop.

  Basic BLOCKs
     A BLOCK by itself (labeled or not) is semantically
     equivalent to a loop that executes once.  Thus you can use
     any of the loop control statements in it to leave or restart
     the block.  (Note that this is NOT true in "eval{}",
     "sub{}", or contrary to popular belief "do{}" blocks, which
     do NOT count as loops.)  The "continue" block is optional.

     The BLOCK construct can be used to emulate case structures.

         SWITCH: {
             if (/^abc/) { $abc = 1; last SWITCH; }
             if (/^def/) { $def = 1; last SWITCH; }
             if (/^xyz/) { $xyz = 1; last SWITCH; }
             $nothing = 1;

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     Such constructs are quite frequently used, because older
     versions of Perl had no official "switch" statement.

  Switch statements
     Starting from Perl 5.10, you can say

         use feature "switch";

     which enables a switch feature that is closely based on the
     Perl 6 proposal.

     The keywords "given" and "when" are analogous to "switch"
     and "case" in other languages, so the code above could be
     written as

         given($_) {
             when (/^abc/) { $abc = 1; }
             when (/^def/) { $def = 1; }
             when (/^xyz/) { $xyz = 1; }
             default { $nothing = 1; }

     This construct is very flexible and powerful. For example:

         use feature ":5.10";
         given($foo) {
             when (undef) {
                 say '$foo is undefined';
             when ("foo") {
                 say '$foo is the string "foo"';
             when ([1,3,5,7,9]) {
                 say '$foo is an odd digit';
                 continue; # Fall through
             when ($_ < 100) {
                 say '$foo is numerically less than 100';
             when (\&complicated_check) {
                 say 'a complicated check for $foo is true';
             default {
                 die q(I don't know what to do with $foo);

     "given(EXPR)" will assign the value of EXPR to $_ within the
     lexical scope of the block, so it's similar to

             do { my $_ = EXPR; ... }

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     except that the block is automatically broken out of by a
     successful "when" or an explicit "break".

     Most of the power comes from implicit smart matching:


     is exactly equivalent to

             when($_ ~~ $foo)

     Most of the time, "when(EXPR)" is treated as an implicit
     smart match of $_, i.e. "$_ ~~ EXPR". (See "Smart matching
     in detail" for more information on smart matching.) But when
     EXPR is one of the below exceptional cases, it is used
     directly as a boolean:

     o   a subroutine or method call

     o   a regular expression match, i.e. "/REGEX/" or "$foo =~
         /REGEX/", or a negated regular expression match
         ("!/REGEX/" or "$foo !~ /REGEX/").

     o   a comparison such as "$_ < 10" or "$x eq "abc"" (or of
         course "$_ ~~ $c")

     o   "defined(...)", "exists(...)", or "eof(...)"

     o   a negated expression "!(...)" or "not (...)", or a
         logical exclusive-or "(...) xor (...)".

     o   a filetest operator, with the exception of "-s", "-M",
         "-A", and "-C", that return numerical values, not
         boolean ones.

     o   the ".." and "..." flip-flop operators.

     In those cases the value of EXPR is used directly as a


     o   If EXPR is "... && ..." or "... and ...", the test is
         applied recursively to both arguments. If both arguments
         pass the test, then the argument is treated as boolean.

     o   If EXPR is "... || ...", "... // ..." or "... or ...",
         the test is applied recursively to the first argument.

     These rules look complicated, but usually they will do what
     you want. For example you could write:

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         when (/^\d+$/ && $_ < 75) { ... }

     Another useful shortcut is that, if you use a literal array
     or hash as the argument to "given", it is turned into a
     reference. So "given(@foo)" is the same as "given(\@foo)",
     for example.

     "default" behaves exactly like "when(1 == 1)", which is to
     say that it always matches.

     Breaking out

     You can use the "break" keyword to break out of the
     enclosing "given" block.  Every "when" block is implicitly
     ended with a "break".


     You can use the "continue" keyword to fall through from one
     case to the next:

         given($foo) {
             when (/x/) { say '$foo contains an x'; continue }
             when (/y/) { say '$foo contains a y' }
             default    { say '$foo does not contain a y' }

     Switching in a loop

     Instead of using "given()", you can use a "foreach()" loop.
     For example, here's one way to count how many times a
     particular string occurs in an array:

         my $count = 0;
         for (@array) {
             when ("foo") { ++$count }
         print "\@array contains $count copies of 'foo'\n";

     At the end of all "when" blocks, there is an implicit
     "next".  You can override that with an explicit "last" if
     you're only interested in the first match.

     This doesn't work if you explicitly specify a loop variable,
     as in "for $item (@array)". You have to use the default
     variable $_. (You can use "for my $_ (@array)".)

     Smart matching in detail

     The behaviour of a smart match depends on what type of thing
     its arguments are. The behaviour is determined by the
     following table: the first row that applies determines the

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     match behaviour (which is thus mostly determined by the type
     of the right operand). Note that the smart match implicitly
     dereferences any non-blessed hash or array ref, so the
     "Hash" and "Array" entries apply in those cases. (For
     blessed references, the "Object" entries apply.)

     Note that the "Matching Code" column is not always an exact
     rendition.  For example, the smart match operator short-
     circuits whenever possible, but "grep" does not.

         $a      $b        Type of Match Implied    Matching Code
         ======  =====     =====================    =============
         Any     undef     undefined                !defined $a

         Any     Object    invokes ~~ overloading on $object, or dies

         Hash    CodeRef   sub truth for each key[1] !grep { !$b->($_) } keys %$a
         Array   CodeRef   sub truth for each elt[1] !grep { !$b->($_) } @$a
         Any     CodeRef   scalar sub truth          $b->($a)

         Hash    Hash      hash keys identical (every key is found in both hashes)
         Array   Hash      hash keys intersection   grep { exists $b->{$_} } @$a
         Regex   Hash      hash key grep            grep /$a/, keys %$b
         undef   Hash      always false (undef can't be a key)
         Any     Hash      hash entry existence     exists $b->{$a}

         Hash    Array     hash keys intersection   grep { exists $a->{$_} } @$b
         Array   Array     arrays are comparable[2]
         Regex   Array     array grep               grep /$a/, @$b
         undef   Array     array contains undef     grep !defined, @$b
         Any     Array     match against an array element[3]
                                                    grep $a ~~ $_, @$b

         Hash    Regex     hash key grep            grep /$b/, keys %$a
         Array   Regex     array grep               grep /$b/, @$a
         Any     Regex     pattern match            $a =~ /$b/

         Object  Any       invokes ~~ overloading on $object, or falls back:
         Any     Num       numeric equality         $a == $b
         Num     numish[4] numeric equality         $a == $b
         undef   Any       undefined                !defined($b)
         Any     Any       string equality          $a eq $b

      1 - empty hashes or arrays will match.
      2 - that is, each element smart-matches the element of same index in the
          other array. [3]
      3 - If a circular reference is found, we fall back to referential equality.
      4 - either a real number, or a string that looks like a number

     Custom matching via overloading

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     You can change the way that an object is matched by
     overloading the "~~" operator. This may alter the usual
     smart match semantics.

     It should be noted that "~~" will refuse to work on objects
     that don't overload it (in order to avoid relying on the
     object's underlying structure).

     Note also that smart match's matching rules take precedence
     over overloading, so if $obj has smart match overloading,

         $obj ~~ X

     will not automatically invoke the overload method with X as
     an argument; instead the table above is consulted as normal,
     and based in the type of X, overloading may or may not be

     See overload.

     Differences from Perl 6

     The Perl 5 smart match and "given"/"when" constructs are not
     absolutely identical to their Perl 6 analogues. The most
     visible difference is that, in Perl 5, parentheses are
     required around the argument to "given()" and "when()"
     (except when this last one is used as a statement modifier).
     Parentheses in Perl 6 are always optional in a control
     construct such as "if()", "while()", or "when()"; they can't
     be made optional in Perl 5 without a great deal of potential
     confusion, because Perl 5 would parse the expression

       given $foo {

     as though the argument to "given" were an element of the
     hash %foo, interpreting the braces as hash-element syntax.

     The table of smart matches is not identical to that proposed
     by the Perl 6 specification, mainly due to the differences
     between Perl 6's and Perl 5's data models.

     In Perl 6, "when()" will always do an implicit smart match
     with its argument, whilst it is convenient in Perl 5 to
     suppress this implicit smart match in certain situations, as
     documented above. (The difference is largely because Perl 5
     does not, even internally, have a boolean type.)

     Although not for the faint of heart, Perl does support a

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     "goto" statement.  There are three forms: "goto"-LABEL,
     "goto"-EXPR, and "goto"-&NAME.  A loop's LABEL is not
     actually a valid target for a "goto"; it's just the name of
     the loop.

     The "goto"-LABEL form finds the statement labeled with LABEL
     and resumes execution there.  It may not be used to go into
     any construct that requires initialization, such as a
     subroutine or a "foreach" loop.  It also can't be used to go
     into a construct that is optimized away.  It can be used to
     go almost anywhere else within the dynamic scope, including
     out of subroutines, but it's usually better to use some
     other construct such as "last" or "die".  The author of Perl
     has never felt the need to use this form of "goto" (in Perl,
     that is--C is another matter).

     The "goto"-EXPR form expects a label name, whose scope will
     be resolved dynamically.  This allows for computed "goto"s
     per FORTRAN, but isn't necessarily recommended if you're
     optimizing for maintainability:

         goto(("FOO", "BAR", "GLARCH")[$i]);

     The "goto"-&NAME form is highly magical, and substitutes a
     call to the named subroutine for the currently running
     subroutine.  This is used by "AUTOLOAD()" subroutines that
     wish to load another subroutine and then pretend that the
     other subroutine had been called in the first place (except
     that any modifications to @_ in the current subroutine are
     propagated to the other subroutine.)  After the "goto", not
     even "caller()" will be able to tell that this routine was
     called first.

     In almost all cases like this, it's usually a far, far
     better idea to use the structured control flow mechanisms of
     "next", "last", or "redo" instead of resorting to a "goto".
     For certain applications, the catch and throw pair of
     "eval{}" and die() for exception processing can also be a
     prudent approach.

  PODs: Embedded Documentation
     Perl has a mechanism for intermixing documentation with
     source code.  While it's expecting the beginning of a new
     statement, if the compiler encounters a line that begins
     with an equal sign and a word, like this

         =head1 Here There Be Pods!

     Then that text and all remaining text up through and
     including a line beginning with "=cut" will be ignored.  The
     format of the intervening text is described in perlpod.

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     This allows you to intermix your source code and your
     documentation text freely, as in

         =item snazzle($)

         The snazzle() function will behave in the most spectacular
         form that you can possibly imagine, not even excepting
         cybernetic pyrotechnics.

         =cut back to the compiler, nuff of this pod stuff!

         sub snazzle($) {
             my $thingie = shift;

     Note that pod translators should look at only paragraphs
     beginning with a pod directive (it makes parsing easier),
     whereas the compiler actually knows to look for pod escapes
     even in the middle of a paragraph.  This means that the
     following secret stuff will be ignored by both the compiler
     and the translators.

         =secret stuff
          warn "Neither POD nor CODE!?"
         =cut back
         print "got $a\n";

     You probably shouldn't rely upon the "warn()" being podded
     out forever.  Not all pod translators are well-behaved in
     this regard, and perhaps the compiler will become pickier.

     One may also use pod directives to quickly comment out a
     section of code.

  Plain Old Comments (Not!)
     Perl can process line directives, much like the C
     preprocessor.  Using this, one can control Perl's idea of
     filenames and line numbers in error or warning messages
     (especially for strings that are processed with "eval()").
     The syntax for this mechanism is the same as for most C
     preprocessors: it matches the regular expression

         # example: '# line 42 "new_filename.plx"'
         /^\#   \s*
           line \s+ (\d+)   \s*
           (?:\s("?)([^"]+)\2)? \s*

     with $1 being the line number for the next line, and $3
     being the optional filename (specified with or without

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     There is a fairly obvious gotcha included with the line
     directive: Debuggers and profilers will only show the last
     source line to appear at a particular line number in a given
     file.  Care should be taken not to cause line number
     collisions in code you'd like to debug later.

     Here are some examples that you should be able to type into
     your command shell:

         % perl
         # line 200 "bzzzt"
         # the `#' on the previous line must be the first char on line
         die 'foo';
         foo at bzzzt line 201.

         % perl
         # line 200 "bzzzt"
         eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
         foo at - line 2001.

         % perl
         eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
         foo at foo bar line 200.

         % perl
         # line 345 "goop"
         eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
         print $@;
         foo at goop line 345.

     See attributes(5) for descriptions of the following

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

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     community source was downloaded from

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

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