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
231 LABEL while (EXPR) BLOCK
232 LABEL while (EXPR) BLOCK continue BLOCK
233 LABEL until (EXPR) BLOCK
234 LABEL until (EXPR) BLOCK continue BLOCK
235 LABEL for (EXPR; EXPR; EXPR) BLOCK
236 LABEL foreach VAR (LIST) BLOCK
237 LABEL foreach VAR (LIST) BLOCK continue BLOCK
238 LABEL BLOCK continue BLOCK
240 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
241 not statements. This means that the curly brackets are I<required>--no
242 dangling statements allowed. If you want to write conditionals without
243 curly brackets there are several other ways to do it. The following
244 all do the same thing:
246 if (!open(FOO)) { die "Can't open $FOO: $!"; }
247 die "Can't open $FOO: $!" unless open(FOO);
248 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
249 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
250 # a bit exotic, that last one
252 The C<if> statement is straightforward. Because BLOCKs are always
253 bounded by curly brackets, there is never any ambiguity about which
254 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
255 the sense of the test is reversed.
257 The C<while> statement executes the block as long as the expression is
258 L<true|/"Truth and Falsehood">.
259 The C<until> statement executes the block as long as the expression is
261 The LABEL is optional, and if present, consists of an identifier followed
262 by a colon. The LABEL identifies the loop for the loop control
263 statements C<next>, C<last>, and C<redo>.
264 If the LABEL is omitted, the loop control statement
265 refers to the innermost enclosing loop. This may include dynamically
266 looking back your call-stack at run time to find the LABEL. Such
267 desperate behavior triggers a warning if you use the C<use warnings>
268 pragma or the B<-w> flag.
270 If there is a C<continue> BLOCK, it is always executed just before the
271 conditional is about to be evaluated again. Thus it can be used to
272 increment a loop variable, even when the loop has been continued via
273 the C<next> statement.
275 Extension modules can also hook into the Perl parser to define new
276 kinds of compound statement. These are introduced by a keyword which
277 the extension recognises, and the syntax following the keyword is
278 defined entirely by the extension. If you are an implementor, see
279 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
280 a module, see the module's documentation for details of the syntax that
284 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
286 The C<next> command starts the next iteration of the loop:
288 LINE: while (<STDIN>) {
289 next LINE if /^#/; # discard comments
293 The C<last> command immediately exits the loop in question. The
294 C<continue> block, if any, is not executed:
296 LINE: while (<STDIN>) {
297 last LINE if /^$/; # exit when done with header
301 The C<redo> command restarts the loop block without evaluating the
302 conditional again. The C<continue> block, if any, is I<not> executed.
303 This command is normally used by programs that want to lie to themselves
304 about what was just input.
306 For example, when processing a file like F</etc/termcap>.
307 If your input lines might end in backslashes to indicate continuation, you
308 want to skip ahead and get the next record.
319 which is Perl short-hand for the more explicitly written version:
321 LINE: while (defined($line = <ARGV>)) {
323 if ($line =~ s/\\$//) {
325 redo LINE unless eof(); # not eof(ARGV)!
330 Note that if there were a C<continue> block on the above code, it would
331 get executed only on lines discarded by the regex (since redo skips the
332 continue block). A continue block is often used to reset line counters
333 or C<?pat?> one-time matches:
335 # inspired by :1,$g/fred/s//WILMA/
337 ?(fred)? && s//WILMA $1 WILMA/;
338 ?(barney)? && s//BETTY $1 BETTY/;
339 ?(homer)? && s//MARGE $1 MARGE/;
341 print "$ARGV $.: $_";
342 close ARGV if eof(); # reset $.
343 reset if eof(); # reset ?pat?
346 If the word C<while> is replaced by the word C<until>, the sense of the
347 test is reversed, but the conditional is still tested before the first
350 The loop control statements don't work in an C<if> or C<unless>, since
351 they aren't loops. You can double the braces to make them such, though.
355 next if /barney/; # same effect as "last", but doesn't document as well
359 This is caused by the fact that a block by itself acts as a loop that
360 executes once, see L<"Basic BLOCKs">.
362 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
363 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
368 Perl's C-style C<for> loop works like the corresponding C<while> loop;
369 that means that this:
371 for ($i = 1; $i < 10; $i++) {
384 There is one minor difference: if variables are declared with C<my>
385 in the initialization section of the C<for>, the lexical scope of
386 those variables is exactly the C<for> loop (the body of the loop
387 and the control sections).
390 Besides the normal array index looping, C<for> can lend itself
391 to many other interesting applications. Here's one that avoids the
392 problem you get into if you explicitly test for end-of-file on
393 an interactive file descriptor causing your program to appear to
395 X<eof> X<end-of-file> X<end of file>
397 $on_a_tty = -t STDIN && -t STDOUT;
398 sub prompt { print "yes? " if $on_a_tty }
399 for ( prompt(); <STDIN>; prompt() ) {
403 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
404 conditional of a C<for> loop is shorthand for the following. This
405 behaviour is the same as a C<while> loop conditional.
406 X<readline> X<< <> >>
408 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
415 The C<foreach> loop iterates over a normal list value and sets the
416 variable VAR to be each element of the list in turn. If the variable
417 is preceded with the keyword C<my>, then it is lexically scoped, and
418 is therefore visible only within the loop. Otherwise, the variable is
419 implicitly local to the loop and regains its former value upon exiting
420 the loop. If the variable was previously declared with C<my>, it uses
421 that variable instead of the global one, but it's still localized to
422 the loop. This implicit localisation occurs I<only> in a C<foreach>
426 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
427 you can use C<foreach> for readability or C<for> for brevity. (Or because
428 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
429 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
432 If any element of LIST is an lvalue, you can modify it by modifying
433 VAR inside the loop. Conversely, if any element of LIST is NOT an
434 lvalue, any attempt to modify that element will fail. In other words,
435 the C<foreach> loop index variable is an implicit alias for each item
436 in the list that you're looping over.
439 If any part of LIST is an array, C<foreach> will get very confused if
440 you add or remove elements within the loop body, for example with
441 C<splice>. So don't do that.
444 C<foreach> probably won't do what you expect if VAR is a tied or other
445 special variable. Don't do that either.
449 for (@ary) { s/foo/bar/ }
451 for my $elem (@elements) {
455 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
456 print $count, "\n"; sleep(1);
459 for (1..15) { print "Merry Christmas\n"; }
461 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
462 print "Item: $item\n";
465 Here's how a C programmer might code up a particular algorithm in Perl:
467 for (my $i = 0; $i < @ary1; $i++) {
468 for (my $j = 0; $j < @ary2; $j++) {
469 if ($ary1[$i] > $ary2[$j]) {
470 last; # can't go to outer :-(
472 $ary1[$i] += $ary2[$j];
474 # this is where that last takes me
477 Whereas here's how a Perl programmer more comfortable with the idiom might
480 OUTER: for my $wid (@ary1) {
481 INNER: for my $jet (@ary2) {
482 next OUTER if $wid > $jet;
487 See how much easier this is? It's cleaner, safer, and faster. It's
488 cleaner because it's less noisy. It's safer because if code gets added
489 between the inner and outer loops later on, the new code won't be
490 accidentally executed. The C<next> explicitly iterates the other loop
491 rather than merely terminating the inner one. And it's faster because
492 Perl executes a C<foreach> statement more rapidly than it would the
493 equivalent C<for> loop.
498 A BLOCK by itself (labeled or not) is semantically equivalent to a
499 loop that executes once. Thus you can use any of the loop control
500 statements in it to leave or restart the block. (Note that this is
501 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
502 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
505 The BLOCK construct can be used to emulate case structures.
508 if (/^abc/) { $abc = 1; last SWITCH; }
509 if (/^def/) { $def = 1; last SWITCH; }
510 if (/^xyz/) { $xyz = 1; last SWITCH; }
514 Such constructs are quite frequently used, because older versions
515 of Perl had no official C<switch> statement.
517 =head2 Switch statements
518 X<switch> X<case> X<given> X<when> X<default>
520 Starting from Perl 5.10, you can say
522 use feature "switch";
524 which enables a switch feature that is closely based on the
527 The keywords C<given> and C<when> are analogous
528 to C<switch> and C<case> in other languages, so the code
529 above could be written as
532 when (/^abc/) { $abc = 1; }
533 when (/^def/) { $def = 1; }
534 when (/^xyz/) { $xyz = 1; }
535 default { $nothing = 1; }
538 This construct is very flexible and powerful. For example:
543 say '$foo is undefined';
546 say '$foo is the string "foo"';
549 say '$foo is an odd digit';
550 continue; # Fall through
553 say '$foo is numerically less than 100';
555 when (\&complicated_check) {
556 say 'a complicated check for $foo is true';
559 die q(I don't know what to do with $foo);
563 C<given(EXPR)> will assign the value of EXPR to C<$_>
564 within the lexical scope of the block, so it's similar to
566 do { my $_ = EXPR; ... }
568 except that the block is automatically broken out of by a
569 successful C<when> or an explicit C<break>.
571 Most of the power comes from implicit smart matching:
575 is exactly equivalent to
579 Most of the time, C<when(EXPR)> is treated as an implicit smart match of
580 C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more
581 information on smart matching.) But when EXPR is one of the below
582 exceptional cases, it is used directly as a boolean:
588 a subroutine or method call
592 a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>,
593 or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>).
597 a comparison such as C<$_ E<lt> 10> or C<$x eq "abc">
598 (or of course C<$_ ~~ $c>)
602 C<defined(...)>, C<exists(...)>, or C<eof(...)>
606 a negated expression C<!(...)> or C<not (...)>, or a logical
607 exclusive-or C<(...) xor (...)>.
611 a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>,
612 that return numerical values, not boolean ones.
616 the C<..> and C<...> flip-flop operators.
620 In those cases the value of EXPR is used directly as a boolean.
628 If EXPR is C<... && ...> or C<... and ...>, the test
629 is applied recursively to both arguments. If I<both>
630 arguments pass the test, then the argument is treated
635 If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test
636 is applied recursively to the first argument.
640 These rules look complicated, but usually they will do what
641 you want. For example you could write:
643 when (/^\d+$/ && $_ < 75) { ... }
645 Another useful shortcut is that, if you use a literal array
646 or hash as the argument to C<given>, it is turned into a
647 reference. So C<given(@foo)> is the same as C<given(\@foo)>,
650 C<default> behaves exactly like C<when(1 == 1)>, which is
651 to say that it always matches.
655 You can use the C<break> keyword to break out of the enclosing
656 C<given> block. Every C<when> block is implicitly ended with
661 You can use the C<continue> keyword to fall through from one
665 when (/x/) { say '$foo contains an x'; continue }
666 when (/y/) { say '$foo contains a y' }
667 default { say '$foo does not contain a y' }
670 =head3 Switching in a loop
672 Instead of using C<given()>, you can use a C<foreach()> loop.
673 For example, here's one way to count how many times a particular
674 string occurs in an array:
678 when ("foo") { ++$count }
680 print "\@array contains $count copies of 'foo'\n";
682 At the end of all C<when> blocks, there is an implicit C<next>.
683 You can override that with an explicit C<last> if you're only
684 interested in the first match.
686 This doesn't work if you explicitly specify a loop variable,
687 as in C<for $item (@array)>. You have to use the default
688 variable C<$_>. (You can use C<for my $_ (@array)>.)
690 =head3 Smart matching in detail
692 The behaviour of a smart match depends on what type of thing its arguments
693 are. The behaviour is determined by the following table: the first row
694 that applies determines the match behaviour (which is thus mostly
695 determined by the type of the right operand). Note that the smart match
696 implicitly dereferences any non-blessed hash or array ref, so the "Hash"
697 and "Array" entries apply in those cases. (For blessed references, the
698 "Object" entries apply.)
700 Note that the "Matching Code" column is not always an exact rendition. For
701 example, the smart match operator short-circuits whenever possible, but
704 $a $b Type of Match Implied Matching Code
705 ====== ===== ===================== =============
706 Any undef undefined !defined $a
708 Any Object invokes ~~ overloading on $object, or dies
710 Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
711 Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
712 Any CodeRef scalar sub truth $b->($a)
714 Hash Hash hash keys identical (every key is found in both hashes)
715 Array Hash hash keys intersection grep { exists $b->{$_} } @$a
716 Regex Hash hash key grep grep /$a/, keys %$b
717 undef Hash always false (undef can't be a key)
718 Any Hash hash entry existence exists $b->{$a}
720 Hash Array hash keys intersection grep { exists $a->{$_} } @$b
721 Array Array arrays are comparable[2]
722 Regex Array array grep grep /$a/, @$b
723 undef Array array contains undef grep !defined, @$b
724 Any Array match against an array element[3]
727 Hash Regex hash key grep grep /$b/, keys %$a
728 Array Regex array grep grep /$b/, @$a
729 Any Regex pattern match $a =~ /$b/
731 Object Any invokes ~~ overloading on $object, or falls back:
732 Any Num numeric equality $a == $b
733 Num numish[4] numeric equality $a == $b
734 undef Any undefined !defined($b)
735 Any Any string equality $a eq $b
737 1 - empty hashes or arrays will match.
738 2 - that is, each element smart-matches the element of same index in the
740 3 - If a circular reference is found, we fall back to referential equality.
741 4 - either a real number, or a string that looks like a number
743 =head3 Custom matching via overloading
745 You can change the way that an object is matched by overloading
746 the C<~~> operator. This may alter the usual smart match semantics.
748 It should be noted that C<~~> will refuse to work on objects that
749 don't overload it (in order to avoid relying on the object's
750 underlying structure).
752 Note also that smart match's matching rules take precedence over
753 overloading, so if C<$obj> has smart match overloading, then
757 will not automatically invoke the overload method with X as an argument;
758 instead the table above is consulted as normal, and based in the type of X,
759 overloading may or may not be invoked.
763 =head3 Differences from Perl 6
765 The Perl 5 smart match and C<given>/C<when> constructs are not
766 absolutely identical to their Perl 6 analogues. The most visible
767 difference is that, in Perl 5, parentheses are required around
768 the argument to C<given()> and C<when()> (except when this last
769 one is used as a statement modifier). Parentheses in Perl 6
770 are always optional in a control construct such as C<if()>,
771 C<while()>, or C<when()>; they can't be made optional in Perl
772 5 without a great deal of potential confusion, because Perl 5
773 would parse the expression
779 as though the argument to C<given> were an element of the hash
780 C<%foo>, interpreting the braces as hash-element syntax.
782 The table of smart matches is not identical to that proposed by the
783 Perl 6 specification, mainly due to the differences between Perl 6's
784 and Perl 5's data models.
786 In Perl 6, C<when()> will always do an implicit smart match
787 with its argument, whilst it is convenient in Perl 5 to
788 suppress this implicit smart match in certain situations,
789 as documented above. (The difference is largely because Perl 5
790 does not, even internally, have a boolean type.)
795 Although not for the faint of heart, Perl does support a C<goto>
796 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
797 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
798 a C<goto>; it's just the name of the loop.
800 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
801 execution there. It may not be used to go into any construct that
802 requires initialization, such as a subroutine or a C<foreach> loop. It
803 also can't be used to go into a construct that is optimized away. It
804 can be used to go almost anywhere else within the dynamic scope,
805 including out of subroutines, but it's usually better to use some other
806 construct such as C<last> or C<die>. The author of Perl has never felt the
807 need to use this form of C<goto> (in Perl, that is--C is another matter).
809 The C<goto>-EXPR form expects a label name, whose scope will be resolved
810 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
811 necessarily recommended if you're optimizing for maintainability:
813 goto(("FOO", "BAR", "GLARCH")[$i]);
815 The C<goto>-&NAME form is highly magical, and substitutes a call to the
816 named subroutine for the currently running subroutine. This is used by
817 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
818 pretend that the other subroutine had been called in the first place
819 (except that any modifications to C<@_> in the current subroutine are
820 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
821 will be able to tell that this routine was called first.
823 In almost all cases like this, it's usually a far, far better idea to use the
824 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
825 resorting to a C<goto>. For certain applications, the catch and throw pair of
826 C<eval{}> and die() for exception processing can also be a prudent approach.
828 =head2 PODs: Embedded Documentation
829 X<POD> X<documentation>
831 Perl has a mechanism for intermixing documentation with source code.
832 While it's expecting the beginning of a new statement, if the compiler
833 encounters a line that begins with an equal sign and a word, like this
835 =head1 Here There Be Pods!
837 Then that text and all remaining text up through and including a line
838 beginning with C<=cut> will be ignored. The format of the intervening
839 text is described in L<perlpod>.
841 This allows you to intermix your source code
842 and your documentation text freely, as in
846 The snazzle() function will behave in the most spectacular
847 form that you can possibly imagine, not even excepting
848 cybernetic pyrotechnics.
850 =cut back to the compiler, nuff of this pod stuff!
857 Note that pod translators should look at only paragraphs beginning
858 with a pod directive (it makes parsing easier), whereas the compiler
859 actually knows to look for pod escapes even in the middle of a
860 paragraph. This means that the following secret stuff will be
861 ignored by both the compiler and the translators.
865 warn "Neither POD nor CODE!?"
869 You probably shouldn't rely upon the C<warn()> being podded out forever.
870 Not all pod translators are well-behaved in this regard, and perhaps
871 the compiler will become pickier.
873 One may also use pod directives to quickly comment out a section
876 =head2 Plain Old Comments (Not!)
877 X<comment> X<line> X<#> X<preprocessor> X<eval>
879 Perl can process line directives, much like the C preprocessor. Using
880 this, one can control Perl's idea of filenames and line numbers in
881 error or warning messages (especially for strings that are processed
882 with C<eval()>). The syntax for this mechanism is the same as for most
883 C preprocessors: it matches the regular expression
885 # example: '# line 42 "new_filename.plx"'
888 (?:\s("?)([^"]+)\2)? \s*
891 with C<$1> being the line number for the next line, and C<$3> being
892 the optional filename (specified with or without quotes).
894 There is a fairly obvious gotcha included with the line directive:
895 Debuggers and profilers will only show the last source line to appear
896 at a particular line number in a given file. Care should be taken not
897 to cause line number collisions in code you'd like to debug later.
899 Here are some examples that you should be able to type into your command
904 # the `#' on the previous line must be the first char on line
907 foo at bzzzt line 201.
911 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
916 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
918 foo at foo bar line 200.
922 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
925 foo at goop line 345.