8 A Perl program consists of a sequence of declarations and statements
9 which run from the top to the bottom. Loops, subroutines and other
10 control structures allow you to jump around within the code.
12 Perl is a B<free-form> language, you can format and indent it however
13 you like. Whitespace mostly serves to separate tokens, unlike
14 languages like Python where it is an important part of the syntax.
16 Many of Perl's syntactic elements are B<optional>. Rather than
17 requiring you to put parentheses around every function call and
18 declare every variable, you can often leave such explicit elements off
19 and Perl will figure out what you meant. This is known as B<Do What I
20 Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
21 code in a style with which they are comfortable.
23 Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
24 Bourne Shell, Smalltalk, Lisp and even English. Other
25 languages have borrowed syntax from Perl, particularly its regular
26 expression extensions. So if you have programmed in another language
27 you will see familiar pieces in Perl. They often work the same, but
28 see L<perltrap> for information about how they differ.
31 X<declaration> X<undef> X<undefined> X<uninitialized>
33 The only things you need to declare in Perl are report formats and
34 subroutines (and sometimes not even subroutines). A variable holds
35 the undefined value (C<undef>) until it has been assigned a defined
36 value, which is anything other than C<undef>. When used as a number,
37 C<undef> is treated as C<0>; when used as a string, it is treated as
38 the empty string, C<"">; and when used as a reference that isn't being
39 assigned to, it is treated as an error. If you enable warnings,
40 you'll be notified of an uninitialized value whenever you treat
41 C<undef> as a string or a number. Well, usually. Boolean contexts,
47 are exempt from warnings (because they care about truth rather than
48 definedness). Operators such as C<++>, C<-->, C<+=>,
49 C<-=>, and C<.=>, that operate on undefined left values such as:
54 are also always exempt from such warnings.
56 A declaration can be put anywhere a statement can, but has no effect on
57 the execution of the primary sequence of statements--declarations all
58 take effect at compile time. Typically all the declarations are put at
59 the beginning or the end of the script. However, if you're using
60 lexically-scoped private variables created with C<my()>, you'll
62 your format or subroutine definition is within the same block scope
63 as the my if you expect to be able to access those private variables.
65 Declaring a subroutine allows a subroutine name to be used as if it were a
66 list operator from that point forward in the program. You can declare a
67 subroutine without defining it by saying C<sub name>, thus:
68 X<subroutine, declaration>
71 $me = myname $0 or die "can't get myname";
73 Note that myname() functions as a list operator, not as a unary operator;
74 so be careful to use C<or> instead of C<||> in this case. However, if
75 you were to declare the subroutine as C<sub myname ($)>, then
76 C<myname> would function as a unary operator, so either C<or> or
79 Subroutines declarations can also be loaded up with the C<require> statement
80 or both loaded and imported into your namespace with a C<use> statement.
81 See L<perlmod> for details on this.
83 A statement sequence may contain declarations of lexically-scoped
84 variables, but apart from declaring a variable name, the declaration acts
85 like an ordinary statement, and is elaborated within the sequence of
86 statements as if it were an ordinary statement. That means it actually
87 has both compile-time and run-time effects.
92 Text from a C<"#"> character until the end of the line is a comment,
93 and is ignored. Exceptions include C<"#"> inside a string or regular
96 =head2 Simple Statements
97 X<statement> X<semicolon> X<expression> X<;>
99 The only kind of simple statement is an expression evaluated for its
100 side effects. Every simple statement must be terminated with a
101 semicolon, unless it is the final statement in a block, in which case
102 the semicolon is optional. (A semicolon is still encouraged if the
103 block takes up more than one line, because you may eventually add
104 another line.) Note that there are some operators like C<eval {}> and
105 C<do {}> that look like compound statements, but aren't (they're just
106 TERMs in an expression), and thus need an explicit termination if used
107 as the last item in a statement.
109 =head2 Truth and Falsehood
110 X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
112 The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
113 C<undef> are all false in a boolean context. All other values are true.
114 Negation of a true value by C<!> or C<not> returns a special false value.
115 When evaluated as a string it is treated as C<''>, but as a number, it
118 =head2 Statement Modifiers
119 X<statement modifier> X<modifier> X<if> X<unless> X<while>
120 X<until> X<when> X<foreach> X<for>
122 Any simple statement may optionally be followed by a I<SINGLE> modifier,
123 just before the terminating semicolon (or block ending). The possible
134 The C<EXPR> following the modifier is referred to as the "condition".
135 Its truth or falsehood determines how the modifier will behave.
137 C<if> executes the statement once I<if> and only if the condition is
138 true. C<unless> is the opposite, it executes the statement I<unless>
139 the condition is true (i.e., if the condition is false).
141 print "Basset hounds got long ears" if length $ear >= 10;
142 go_outside() and play() unless $is_raining;
144 C<when> executes the statement I<when> C<$_> smart matches C<EXPR>, and
145 then either C<break>s out if it's enclosed in a C<given> scope or skips
146 to the C<next> element when it lies directly inside a C<for> loop.
147 See also L</"Switch statements">.
150 $abc = 1 when /^abc/;
151 $just_a = 1 when /^a/;
156 admin($_) when [ qw/Alice Bob/ ];
157 regular($_) when [ qw/Chris David Ellen/ ];
160 The C<foreach> modifier is an iterator: it executes the statement once
161 for each item in the LIST (with C<$_> aliased to each item in turn).
163 print "Hello $_!\n" foreach qw(world Dolly nurse);
165 C<while> repeats the statement I<while> the condition is true.
166 C<until> does the opposite, it repeats the statement I<until> the
167 condition is true (or while the condition is false):
169 # Both of these count from 0 to 10.
170 print $i++ while $i <= 10;
171 print $j++ until $j > 10;
173 The C<while> and C<until> modifiers have the usual "C<while> loop"
174 semantics (conditional evaluated first), except when applied to a
175 C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
176 which case the block executes once before the conditional is
177 evaluated. This is so that you can write loops like:
182 } until $line eq ".\n";
184 See L<perlfunc/do>. Note also that the loop control statements described
185 later will I<NOT> work in this construct, because modifiers don't take
186 loop labels. Sorry. You can always put another block inside of it
187 (for C<next>) or around it (for C<last>) to do that sort of thing.
188 For C<next>, just double the braces:
189 X<next> X<last> X<redo>
196 For C<last>, you have to be more elaborate:
206 B<NOTE:> The behaviour of a C<my> statement modified with a statement
207 modifier conditional or loop construct (e.g. C<my $x if ...>) is
208 B<undefined>. The value of the C<my> variable may be C<undef>, any
209 previously assigned value, or possibly anything else. Don't rely on
210 it. Future versions of perl might do something different from the
211 version of perl you try it out on. Here be dragons.
214 =head2 Compound Statements
215 X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
216 X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue>
218 In Perl, a sequence of statements that defines a scope is called a block.
219 Sometimes a block is delimited by the file containing it (in the case
220 of a required file, or the program as a whole), and sometimes a block
221 is delimited by the extent of a string (in the case of an eval).
223 But generally, a block is delimited by curly brackets, also known as braces.
224 We will call this syntactic construct a BLOCK.
226 The following compound statements may be used to control flow:
229 if (EXPR) BLOCK else BLOCK
230 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
232 unless (EXPR) BLOCK else BLOCK
233 LABEL while (EXPR) BLOCK
234 LABEL while (EXPR) BLOCK continue BLOCK
235 LABEL until (EXPR) BLOCK
236 LABEL until (EXPR) BLOCK continue BLOCK
237 LABEL for (EXPR; EXPR; EXPR) BLOCK
238 LABEL foreach VAR (LIST) BLOCK
239 LABEL foreach VAR (LIST) BLOCK continue BLOCK
240 LABEL BLOCK continue BLOCK
242 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
243 not statements. This means that the curly brackets are I<required>--no
244 dangling statements allowed. If you want to write conditionals without
245 curly brackets there are several other ways to do it. The following
246 all do the same thing:
248 if (!open(FOO)) { die "Can't open $FOO: $!"; }
249 die "Can't open $FOO: $!" unless open(FOO);
250 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
251 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
252 # a bit exotic, that last one
254 The C<if> statement is straightforward. Because BLOCKs are always
255 bounded by curly brackets, there is never any ambiguity about which
256 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
257 the sense of the test is reversed. In Perl even C<unless> followed
260 The C<while> statement executes the block as long as the expression is
261 L<true|/"Truth and Falsehood">.
262 The C<until> statement executes the block as long as the expression is
264 The LABEL is optional, and if present, consists of an identifier followed
265 by a colon. The LABEL identifies the loop for the loop control
266 statements C<next>, C<last>, and C<redo>.
267 If the LABEL is omitted, the loop control statement
268 refers to the innermost enclosing loop. This may include dynamically
269 looking back your call-stack at run time to find the LABEL. Such
270 desperate behavior triggers a warning if you use the C<use warnings>
271 pragma or the B<-w> flag.
273 If there is a C<continue> BLOCK, it is always executed just before the
274 conditional is about to be evaluated again. Thus it can be used to
275 increment a loop variable, even when the loop has been continued via
276 the C<next> statement.
278 Extension modules can also hook into the Perl parser to define new
279 kinds of compound statement. These are introduced by a keyword which
280 the extension recognises, and the syntax following the keyword is
281 defined entirely by the extension. If you are an implementor, see
282 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
283 a module, see the module's documentation for details of the syntax that
287 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
289 The C<next> command starts the next iteration of the loop:
291 LINE: while (<STDIN>) {
292 next LINE if /^#/; # discard comments
296 The C<last> command immediately exits the loop in question. The
297 C<continue> block, if any, is not executed:
299 LINE: while (<STDIN>) {
300 last LINE if /^$/; # exit when done with header
304 The C<redo> command restarts the loop block without evaluating the
305 conditional again. The C<continue> block, if any, is I<not> executed.
306 This command is normally used by programs that want to lie to themselves
307 about what was just input.
309 For example, when processing a file like F</etc/termcap>.
310 If your input lines might end in backslashes to indicate continuation, you
311 want to skip ahead and get the next record.
322 which is Perl short-hand for the more explicitly written version:
324 LINE: while (defined($line = <ARGV>)) {
326 if ($line =~ s/\\$//) {
328 redo LINE unless eof(); # not eof(ARGV)!
333 Note that if there were a C<continue> block on the above code, it would
334 get executed only on lines discarded by the regex (since redo skips the
335 continue block). A continue block is often used to reset line counters
336 or C<?pat?> one-time matches:
338 # inspired by :1,$g/fred/s//WILMA/
340 ?(fred)? && s//WILMA $1 WILMA/;
341 ?(barney)? && s//BETTY $1 BETTY/;
342 ?(homer)? && s//MARGE $1 MARGE/;
344 print "$ARGV $.: $_";
345 close ARGV if eof(); # reset $.
346 reset if eof(); # reset ?pat?
349 If the word C<while> is replaced by the word C<until>, the sense of the
350 test is reversed, but the conditional is still tested before the first
353 The loop control statements don't work in an C<if> or C<unless>, since
354 they aren't loops. You can double the braces to make them such, though.
358 next if /barney/; # same effect as "last", but doesn't document as well
362 This is caused by the fact that a block by itself acts as a loop that
363 executes once, see L<"Basic BLOCKs">.
365 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
366 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
371 Perl's C-style C<for> loop works like the corresponding C<while> loop;
372 that means that this:
374 for ($i = 1; $i < 10; $i++) {
387 There is one minor difference: if variables are declared with C<my>
388 in the initialization section of the C<for>, the lexical scope of
389 those variables is exactly the C<for> loop (the body of the loop
390 and the control sections).
393 Besides the normal array index looping, C<for> can lend itself
394 to many other interesting applications. Here's one that avoids the
395 problem you get into if you explicitly test for end-of-file on
396 an interactive file descriptor causing your program to appear to
398 X<eof> X<end-of-file> X<end of file>
400 $on_a_tty = -t STDIN && -t STDOUT;
401 sub prompt { print "yes? " if $on_a_tty }
402 for ( prompt(); <STDIN>; prompt() ) {
406 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
407 conditional of a C<for> loop is shorthand for the following. This
408 behaviour is the same as a C<while> loop conditional.
409 X<readline> X<< <> >>
411 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
418 The C<foreach> loop iterates over a normal list value and sets the
419 variable VAR to be each element of the list in turn. If the variable
420 is preceded with the keyword C<my>, then it is lexically scoped, and
421 is therefore visible only within the loop. Otherwise, the variable is
422 implicitly local to the loop and regains its former value upon exiting
423 the loop. If the variable was previously declared with C<my>, it uses
424 that variable instead of the global one, but it's still localized to
425 the loop. This implicit localisation occurs I<only> in a C<foreach>
429 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
430 you can use C<foreach> for readability or C<for> for brevity. (Or because
431 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
432 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
435 If any element of LIST is an lvalue, you can modify it by modifying
436 VAR inside the loop. Conversely, if any element of LIST is NOT an
437 lvalue, any attempt to modify that element will fail. In other words,
438 the C<foreach> loop index variable is an implicit alias for each item
439 in the list that you're looping over.
442 If any part of LIST is an array, C<foreach> will get very confused if
443 you add or remove elements within the loop body, for example with
444 C<splice>. So don't do that.
447 C<foreach> probably won't do what you expect if VAR is a tied or other
448 special variable. Don't do that either.
452 for (@ary) { s/foo/bar/ }
454 for my $elem (@elements) {
458 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
459 print $count, "\n"; sleep(1);
462 for (1..15) { print "Merry Christmas\n"; }
464 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
465 print "Item: $item\n";
468 Here's how a C programmer might code up a particular algorithm in Perl:
470 for (my $i = 0; $i < @ary1; $i++) {
471 for (my $j = 0; $j < @ary2; $j++) {
472 if ($ary1[$i] > $ary2[$j]) {
473 last; # can't go to outer :-(
475 $ary1[$i] += $ary2[$j];
477 # this is where that last takes me
480 Whereas here's how a Perl programmer more comfortable with the idiom might
483 OUTER: for my $wid (@ary1) {
484 INNER: for my $jet (@ary2) {
485 next OUTER if $wid > $jet;
490 See how much easier this is? It's cleaner, safer, and faster. It's
491 cleaner because it's less noisy. It's safer because if code gets added
492 between the inner and outer loops later on, the new code won't be
493 accidentally executed. The C<next> explicitly iterates the other loop
494 rather than merely terminating the inner one. And it's faster because
495 Perl executes a C<foreach> statement more rapidly than it would the
496 equivalent C<for> loop.
501 A BLOCK by itself (labeled or not) is semantically equivalent to a
502 loop that executes once. Thus you can use any of the loop control
503 statements in it to leave or restart the block. (Note that this is
504 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
505 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
508 The BLOCK construct can be used to emulate case structures.
511 if (/^abc/) { $abc = 1; last SWITCH; }
512 if (/^def/) { $def = 1; last SWITCH; }
513 if (/^xyz/) { $xyz = 1; last SWITCH; }
517 Such constructs are quite frequently used, because older versions
518 of Perl had no official C<switch> statement.
520 =head2 Switch statements
521 X<switch> X<case> X<given> X<when> X<default>
523 Starting from Perl 5.10, you can say
525 use feature "switch";
527 which enables a switch feature that is closely based on the
530 The keywords C<given> and C<when> are analogous
531 to C<switch> and C<case> in other languages, so the code
532 above could be written as
535 when (/^abc/) { $abc = 1; }
536 when (/^def/) { $def = 1; }
537 when (/^xyz/) { $xyz = 1; }
538 default { $nothing = 1; }
541 This construct is very flexible and powerful. For example:
546 say '$foo is undefined';
549 say '$foo is the string "foo"';
552 say '$foo is an odd digit';
553 continue; # Fall through
556 say '$foo is numerically less than 100';
558 when (\&complicated_check) {
559 say 'a complicated check for $foo is true';
562 die q(I don't know what to do with $foo);
566 C<given(EXPR)> will assign the value of EXPR to C<$_>
567 within the lexical scope of the block, so it's similar to
569 do { my $_ = EXPR; ... }
571 except that the block is automatically broken out of by a
572 successful C<when> or an explicit C<break>.
574 Most of the power comes from implicit smart matching:
578 is exactly equivalent to
582 Most of the time, C<when(EXPR)> is treated as an implicit smart match of
583 C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more
584 information on smart matching.) But when EXPR is one of the below
585 exceptional cases, it is used directly as a boolean:
591 a subroutine or method call
595 a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>,
596 or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>).
600 a comparison such as C<$_ E<lt> 10> or C<$x eq "abc">
601 (or of course C<$_ ~~ $c>)
605 C<defined(...)>, C<exists(...)>, or C<eof(...)>
609 a negated expression C<!(...)> or C<not (...)>, or a logical
610 exclusive-or C<(...) xor (...)>.
614 a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>,
615 that return numerical values, not boolean ones.
619 the C<..> and C<...> flip-flop operators.
623 In those cases the value of EXPR is used directly as a boolean.
631 If EXPR is C<... && ...> or C<... and ...>, the test
632 is applied recursively to both arguments. If I<both>
633 arguments pass the test, then the argument is treated
638 If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test
639 is applied recursively to the first argument.
643 These rules look complicated, but usually they will do what
644 you want. For example you could write:
646 when (/^\d+$/ && $_ < 75) { ... }
648 Another useful shortcut is that, if you use a literal array
649 or hash as the argument to C<given>, it is turned into a
650 reference. So C<given(@foo)> is the same as C<given(\@foo)>,
653 C<default> behaves exactly like C<when(1 == 1)>, which is
654 to say that it always matches.
658 You can use the C<break> keyword to break out of the enclosing
659 C<given> block. Every C<when> block is implicitly ended with
664 You can use the C<continue> keyword to fall through from one
668 when (/x/) { say '$foo contains an x'; continue }
669 when (/y/) { say '$foo contains a y' }
670 default { say '$foo does not contain a y' }
673 =head3 Switching in a loop
675 Instead of using C<given()>, you can use a C<foreach()> loop.
676 For example, here's one way to count how many times a particular
677 string occurs in an array:
681 when ("foo") { ++$count }
683 print "\@array contains $count copies of 'foo'\n";
685 At the end of all C<when> blocks, there is an implicit C<next>.
686 You can override that with an explicit C<last> if you're only
687 interested in the first match.
689 This doesn't work if you explicitly specify a loop variable,
690 as in C<for $item (@array)>. You have to use the default
691 variable C<$_>. (You can use C<for my $_ (@array)>.)
693 =head3 Smart matching in detail
695 The behaviour of a smart match depends on what type of thing its arguments
696 are. The behaviour is determined by the following table: the first row
697 that applies determines the match behaviour (which is thus mostly
698 determined by the type of the right operand). Note that the smart match
699 implicitly dereferences any non-blessed hash or array ref, so the "Hash"
700 and "Array" entries apply in those cases. (For blessed references, the
701 "Object" entries apply.)
703 Note that the "Matching Code" column is not always an exact rendition. For
704 example, the smart match operator short-circuits whenever possible, but
707 $a $b Type of Match Implied Matching Code
708 ====== ===== ===================== =============
709 Any undef undefined !defined $a
711 Any Object invokes ~~ overloading on $object, or dies
713 Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
714 Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
715 Any CodeRef scalar sub truth $b->($a)
717 Hash Hash hash keys identical (every key is found in both hashes)
718 Array Hash hash keys intersection grep { exists $b->{$_} } @$a
719 Regex Hash hash key grep grep /$a/, keys %$b
720 undef Hash always false (undef can't be a key)
721 Any Hash hash entry existence exists $b->{$a}
723 Hash Array hash keys intersection grep { exists $a->{$_} } @$b
724 Array Array arrays are comparable[2]
725 Regex Array array grep grep /$a/, @$b
726 undef Array array contains undef grep !defined, @$b
727 Any Array match against an array element[3]
730 Hash Regex hash key grep grep /$b/, keys %$a
731 Array Regex array grep grep /$b/, @$a
732 Any Regex pattern match $a =~ /$b/
734 Object Any invokes ~~ overloading on $object, or falls back:
735 Any Num numeric equality $a == $b
736 Num numish[4] numeric equality $a == $b
737 undef Any undefined !defined($b)
738 Any Any string equality $a eq $b
740 1 - empty hashes or arrays will match.
741 2 - that is, each element smart-matches the element of same index in the
743 3 - If a circular reference is found, we fall back to referential equality.
744 4 - either a real number, or a string that looks like a number
746 =head3 Custom matching via overloading
748 You can change the way that an object is matched by overloading
749 the C<~~> operator. This may alter the usual smart match semantics.
751 It should be noted that C<~~> will refuse to work on objects that
752 don't overload it (in order to avoid relying on the object's
753 underlying structure).
755 Note also that smart match's matching rules take precedence over
756 overloading, so if C<$obj> has smart match overloading, then
760 will not automatically invoke the overload method with X as an argument;
761 instead the table above is consulted as normal, and based in the type of X,
762 overloading may or may not be invoked.
766 =head3 Differences from Perl 6
768 The Perl 5 smart match and C<given>/C<when> constructs are not
769 absolutely identical to their Perl 6 analogues. The most visible
770 difference is that, in Perl 5, parentheses are required around
771 the argument to C<given()> and C<when()> (except when this last
772 one is used as a statement modifier). Parentheses in Perl 6
773 are always optional in a control construct such as C<if()>,
774 C<while()>, or C<when()>; they can't be made optional in Perl
775 5 without a great deal of potential confusion, because Perl 5
776 would parse the expression
782 as though the argument to C<given> were an element of the hash
783 C<%foo>, interpreting the braces as hash-element syntax.
785 The table of smart matches is not identical to that proposed by the
786 Perl 6 specification, mainly due to the differences between Perl 6's
787 and Perl 5's data models.
789 In Perl 6, C<when()> will always do an implicit smart match
790 with its argument, whilst it is convenient in Perl 5 to
791 suppress this implicit smart match in certain situations,
792 as documented above. (The difference is largely because Perl 5
793 does not, even internally, have a boolean type.)
798 Although not for the faint of heart, Perl does support a C<goto>
799 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
800 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
801 a C<goto>; it's just the name of the loop.
803 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
804 execution there. It may not be used to go into any construct that
805 requires initialization, such as a subroutine or a C<foreach> loop. It
806 also can't be used to go into a construct that is optimized away. It
807 can be used to go almost anywhere else within the dynamic scope,
808 including out of subroutines, but it's usually better to use some other
809 construct such as C<last> or C<die>. The author of Perl has never felt the
810 need to use this form of C<goto> (in Perl, that is--C is another matter).
812 The C<goto>-EXPR form expects a label name, whose scope will be resolved
813 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
814 necessarily recommended if you're optimizing for maintainability:
816 goto(("FOO", "BAR", "GLARCH")[$i]);
818 The C<goto>-&NAME form is highly magical, and substitutes a call to the
819 named subroutine for the currently running subroutine. This is used by
820 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
821 pretend that the other subroutine had been called in the first place
822 (except that any modifications to C<@_> in the current subroutine are
823 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
824 will be able to tell that this routine was called first.
826 In almost all cases like this, it's usually a far, far better idea to use the
827 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
828 resorting to a C<goto>. For certain applications, the catch and throw pair of
829 C<eval{}> and die() for exception processing can also be a prudent approach.
831 =head2 PODs: Embedded Documentation
832 X<POD> X<documentation>
834 Perl has a mechanism for intermixing documentation with source code.
835 While it's expecting the beginning of a new statement, if the compiler
836 encounters a line that begins with an equal sign and a word, like this
838 =head1 Here There Be Pods!
840 Then that text and all remaining text up through and including a line
841 beginning with C<=cut> will be ignored. The format of the intervening
842 text is described in L<perlpod>.
844 This allows you to intermix your source code
845 and your documentation text freely, as in
849 The snazzle() function will behave in the most spectacular
850 form that you can possibly imagine, not even excepting
851 cybernetic pyrotechnics.
853 =cut back to the compiler, nuff of this pod stuff!
860 Note that pod translators should look at only paragraphs beginning
861 with a pod directive (it makes parsing easier), whereas the compiler
862 actually knows to look for pod escapes even in the middle of a
863 paragraph. This means that the following secret stuff will be
864 ignored by both the compiler and the translators.
868 warn "Neither POD nor CODE!?"
872 You probably shouldn't rely upon the C<warn()> being podded out forever.
873 Not all pod translators are well-behaved in this regard, and perhaps
874 the compiler will become pickier.
876 One may also use pod directives to quickly comment out a section
879 =head2 Plain Old Comments (Not!)
880 X<comment> X<line> X<#> X<preprocessor> X<eval>
882 Perl can process line directives, much like the C preprocessor. Using
883 this, one can control Perl's idea of filenames and line numbers in
884 error or warning messages (especially for strings that are processed
885 with C<eval()>). The syntax for this mechanism is the same as for most
886 C preprocessors: it matches the regular expression
888 # example: '# line 42 "new_filename.plx"'
891 (?:\s("?)([^"]+)\2)? \s*
894 with C<$1> being the line number for the next line, and C<$3> being
895 the optional filename (specified with or without quotes).
897 There is a fairly obvious gotcha included with the line directive:
898 Debuggers and profilers will only show the last source line to appear
899 at a particular line number in a given file. Care should be taken not
900 to cause line number collisions in code you'd like to debug later.
902 Here are some examples that you should be able to type into your command
907 # the `#' on the previous line must be the first char on line
910 foo at bzzzt line 201.
914 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
919 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
921 foo at foo bar line 200.
925 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
928 foo at goop line 345.