7 A Perl program consists of a sequence of declarations and statements
8 which run from the top to the bottom. Loops, subroutines and other
9 control structures allow you to jump around within the code.
11 Perl is a B<free-form> language, you can format and indent it however
12 you like. Whitespace mostly serves to separate tokens, unlike
13 languages like Python where it is an important part of the syntax.
15 Many of Perl's syntactic elements are B<optional>. Rather than
16 requiring you to put parentheses around every function call and
17 declare every variable, you can often leave such explicit elements off
18 and Perl will figure out what you meant. This is known as B<Do What I
19 Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
20 code in a style with which they are comfortable.
22 Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
23 Bourne Shell, Smalltalk, Lisp and even English. Other
24 languages have borrowed syntax from Perl, particularly its regular
25 expression extensions. So if you have programmed in another language
26 you will see familiar pieces in Perl. They often work the same, but
27 see L<perltrap> for information about how they differ.
31 The only things you need to declare in Perl are report formats and
32 subroutines (and sometimes not even subroutines). A variable holds
33 the undefined value (C<undef>) until it has been assigned a defined
34 value, which is anything other than C<undef>. When used as a number,
35 C<undef> is treated as C<0>; when used as a string, it is treated as
36 the empty string, C<"">; and when used as a reference that isn't being
37 assigned to, it is treated as an error. If you enable warnings,
38 you'll be notified of an uninitialized value whenever you treat
39 C<undef> as a string or a number. Well, usually. Boolean contexts,
45 are exempt from warnings (because they care about truth rather than
46 definedness). Operators such as C<++>, C<-->, C<+=>,
47 C<-=>, and C<.=>, that operate on undefined left values such as:
52 are also always exempt from such warnings.
54 A declaration can be put anywhere a statement can, but has no effect on
55 the execution of the primary sequence of statements--declarations all
56 take effect at compile time. Typically all the declarations are put at
57 the beginning or the end of the script. However, if you're using
58 lexically-scoped private variables created with C<my()>, you'll
60 your format or subroutine definition is within the same block scope
61 as the my if you expect to be able to access those private variables.
63 Declaring a subroutine allows a subroutine name to be used as if it were a
64 list operator from that point forward in the program. You can declare a
65 subroutine without defining it by saying C<sub name>, thus:
68 $me = myname $0 or die "can't get myname";
70 Note that myname() functions as a list operator, not as a unary operator;
71 so be careful to use C<or> instead of C<||> in this case. However, if
72 you were to declare the subroutine as C<sub myname ($)>, then
73 C<myname> would function as a unary operator, so either C<or> or
76 Subroutines declarations can also be loaded up with the C<require> statement
77 or both loaded and imported into your namespace with a C<use> statement.
78 See L<perlmod> for details on this.
80 A statement sequence may contain declarations of lexically-scoped
81 variables, but apart from declaring a variable name, the declaration acts
82 like an ordinary statement, and is elaborated within the sequence of
83 statements as if it were an ordinary statement. That means it actually
84 has both compile-time and run-time effects.
88 Text from a C<"#"> character until the end of the line is a comment,
89 and is ignored. Exceptions include C<"#"> inside a string or regular
92 =head2 Simple Statements
94 The only kind of simple statement is an expression evaluated for its
95 side effects. Every simple statement must be terminated with a
96 semicolon, unless it is the final statement in a block, in which case
97 the semicolon is optional. (A semicolon is still encouraged if the
98 block takes up more than one line, because you may eventually add
99 another line.) Note that there are some operators like C<eval {}> and
100 C<do {}> that look like compound statements, but aren't (they're just
101 TERMs in an expression), and thus need an explicit termination if used
102 as the last item in a statement.
104 =head2 Truth and Falsehood
106 The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
107 C<undef> are all false in a boolean context. All other values are true.
108 Negation of a true value by C<!> or C<not> returns a special false value.
109 When evaluated as a string it is treated as C<''>, but as a number, it
112 =head2 Statement Modifiers
114 Any simple statement may optionally be followed by a I<SINGLE> modifier,
115 just before the terminating semicolon (or block ending). The possible
124 The C<EXPR> following the modifier is referred to as the "condition".
125 Its truth or falsehood determines how the modifier will behave.
127 C<if> executes the statement once I<if> and only if the condition is
128 true. C<unless> is the opposite, it executes the statement I<unless>
129 the condition is true (i.e., if the condition is false).
131 print "Basset hounds got long ears" if length $ear >= 10;
132 go_outside() and play() unless $is_raining;
134 The C<foreach> modifier is an iterator: it executes the statement once
135 for each item in the LIST (with C<$_> aliased to each item in turn).
137 print "Hello $_!\n" foreach qw(world Dolly nurse);
139 C<while> repeats the statement I<while> the condition is true.
140 C<until> does the opposite, it repeats the statement I<until> the
141 condition is true (or while the condition is false):
143 # Both of these count from 0 to 10.
144 print $i++ while $i <= 10;
145 print $j++ until $j > 10;
147 The C<while> and C<until> modifiers have the usual "C<while> loop"
148 semantics (conditional evaluated first), except when applied to a
149 C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
150 which case the block executes once before the conditional is
151 evaluated. This is so that you can write loops like:
156 } until $line eq ".\n";
158 See L<perlfunc/do>. Note also that the loop control statements described
159 later will I<NOT> work in this construct, because modifiers don't take
160 loop labels. Sorry. You can always put another block inside of it
161 (for C<next>) or around it (for C<last>) to do that sort of thing.
162 For C<next>, just double the braces:
169 For C<last>, you have to be more elaborate:
178 B<NOTE:> The behaviour of a C<my> statement modified with a statement
179 modifier conditional or loop construct (e.g. C<my $x if ...>) is
180 B<undefined>. The value of the C<my> variable may be C<undef>, any
181 previously assigned value, or possibly anything else. Don't rely on
182 it. Future versions of perl might do something different from the
183 version of perl you try it out on. Here be dragons.
185 =head2 Compound Statements
187 In Perl, a sequence of statements that defines a scope is called a block.
188 Sometimes a block is delimited by the file containing it (in the case
189 of a required file, or the program as a whole), and sometimes a block
190 is delimited by the extent of a string (in the case of an eval).
192 But generally, a block is delimited by curly brackets, also known as braces.
193 We will call this syntactic construct a BLOCK.
195 The following compound statements may be used to control flow:
198 if (EXPR) BLOCK else BLOCK
199 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
200 LABEL while (EXPR) BLOCK
201 LABEL while (EXPR) BLOCK continue BLOCK
202 LABEL until (EXPR) BLOCK
203 LABEL until (EXPR) BLOCK continue BLOCK
204 LABEL for (EXPR; EXPR; EXPR) BLOCK
205 LABEL foreach VAR (LIST) BLOCK
206 LABEL foreach VAR (LIST) BLOCK continue BLOCK
207 LABEL BLOCK continue BLOCK
209 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
210 not statements. This means that the curly brackets are I<required>--no
211 dangling statements allowed. If you want to write conditionals without
212 curly brackets there are several other ways to do it. The following
213 all do the same thing:
215 if (!open(FOO)) { die "Can't open $FOO: $!"; }
216 die "Can't open $FOO: $!" unless open(FOO);
217 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
218 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
219 # a bit exotic, that last one
221 The C<if> statement is straightforward. Because BLOCKs are always
222 bounded by curly brackets, there is never any ambiguity about which
223 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
224 the sense of the test is reversed.
226 The C<while> statement executes the block as long as the expression is
227 true (does not evaluate to the null string C<""> or C<0> or C<"0">).
228 The LABEL is optional, and if present, consists of an identifier followed
229 by a colon. The LABEL identifies the loop for the loop control
230 statements C<next>, C<last>, and C<redo>.
231 If the LABEL is omitted, the loop control statement
232 refers to the innermost enclosing loop. This may include dynamically
233 looking back your call-stack at run time to find the LABEL. Such
234 desperate behavior triggers a warning if you use the C<use warnings>
235 pragma or the B<-w> flag.
237 If there is a C<continue> BLOCK, it is always executed just before the
238 conditional is about to be evaluated again. Thus it can be used to
239 increment a loop variable, even when the loop has been continued via
240 the C<next> statement.
244 The C<next> command starts the next iteration of the loop:
246 LINE: while (<STDIN>) {
247 next LINE if /^#/; # discard comments
251 The C<last> command immediately exits the loop in question. The
252 C<continue> block, if any, is not executed:
254 LINE: while (<STDIN>) {
255 last LINE if /^$/; # exit when done with header
259 The C<redo> command restarts the loop block without evaluating the
260 conditional again. The C<continue> block, if any, is I<not> executed.
261 This command is normally used by programs that want to lie to themselves
262 about what was just input.
264 For example, when processing a file like F</etc/termcap>.
265 If your input lines might end in backslashes to indicate continuation, you
266 want to skip ahead and get the next record.
277 which is Perl short-hand for the more explicitly written version:
279 LINE: while (defined($line = <ARGV>)) {
281 if ($line =~ s/\\$//) {
283 redo LINE unless eof(); # not eof(ARGV)!
288 Note that if there were a C<continue> block on the above code, it would
289 get executed only on lines discarded by the regex (since redo skips the
290 continue block). A continue block is often used to reset line counters
291 or C<?pat?> one-time matches:
293 # inspired by :1,$g/fred/s//WILMA/
295 ?(fred)? && s//WILMA $1 WILMA/;
296 ?(barney)? && s//BETTY $1 BETTY/;
297 ?(homer)? && s//MARGE $1 MARGE/;
299 print "$ARGV $.: $_";
300 close ARGV if eof(); # reset $.
301 reset if eof(); # reset ?pat?
304 If the word C<while> is replaced by the word C<until>, the sense of the
305 test is reversed, but the conditional is still tested before the first
308 The loop control statements don't work in an C<if> or C<unless>, since
309 they aren't loops. You can double the braces to make them such, though.
313 next if /barney/; # same effect as "last", but doesn't document as well
317 This is caused by the fact that a block by itself acts as a loop that
318 executes once, see L<"Basic BLOCKs and Switch Statements">.
320 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
321 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
325 Perl's C-style C<for> loop works like the corresponding C<while> loop;
326 that means that this:
328 for ($i = 1; $i < 10; $i++) {
341 There is one minor difference: if variables are declared with C<my>
342 in the initialization section of the C<for>, the lexical scope of
343 those variables is exactly the C<for> loop (the body of the loop
344 and the control sections).
346 Besides the normal array index looping, C<for> can lend itself
347 to many other interesting applications. Here's one that avoids the
348 problem you get into if you explicitly test for end-of-file on
349 an interactive file descriptor causing your program to appear to
352 $on_a_tty = -t STDIN && -t STDOUT;
353 sub prompt { print "yes? " if $on_a_tty }
354 for ( prompt(); <STDIN>; prompt() ) {
358 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
359 conditional of a C<for> loop is shorthand for the following. This
360 behaviour is the same as a C<while> loop conditional.
362 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
368 The C<foreach> loop iterates over a normal list value and sets the
369 variable VAR to be each element of the list in turn. If the variable
370 is preceded with the keyword C<my>, then it is lexically scoped, and
371 is therefore visible only within the loop. Otherwise, the variable is
372 implicitly local to the loop and regains its former value upon exiting
373 the loop. If the variable was previously declared with C<my>, it uses
374 that variable instead of the global one, but it's still localized to
375 the loop. This implicit localisation occurs I<only> in a C<foreach>
378 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
379 you can use C<foreach> for readability or C<for> for brevity. (Or because
380 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
381 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
383 If any element of LIST is an lvalue, you can modify it by modifying
384 VAR inside the loop. Conversely, if any element of LIST is NOT an
385 lvalue, any attempt to modify that element will fail. In other words,
386 the C<foreach> loop index variable is an implicit alias for each item
387 in the list that you're looping over.
389 If any part of LIST is an array, C<foreach> will get very confused if
390 you add or remove elements within the loop body, for example with
391 C<splice>. So don't do that.
393 C<foreach> probably won't do what you expect if VAR is a tied or other
394 special variable. Don't do that either.
398 for (@ary) { s/foo/bar/ }
400 for my $elem (@elements) {
404 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
405 print $count, "\n"; sleep(1);
408 for (1..15) { print "Merry Christmas\n"; }
410 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
411 print "Item: $item\n";
414 Here's how a C programmer might code up a particular algorithm in Perl:
416 for (my $i = 0; $i < @ary1; $i++) {
417 for (my $j = 0; $j < @ary2; $j++) {
418 if ($ary1[$i] > $ary2[$j]) {
419 last; # can't go to outer :-(
421 $ary1[$i] += $ary2[$j];
423 # this is where that last takes me
426 Whereas here's how a Perl programmer more comfortable with the idiom might
429 OUTER: for my $wid (@ary1) {
430 INNER: for my $jet (@ary2) {
431 next OUTER if $wid > $jet;
436 See how much easier this is? It's cleaner, safer, and faster. It's
437 cleaner because it's less noisy. It's safer because if code gets added
438 between the inner and outer loops later on, the new code won't be
439 accidentally executed. The C<next> explicitly iterates the other loop
440 rather than merely terminating the inner one. And it's faster because
441 Perl executes a C<foreach> statement more rapidly than it would the
442 equivalent C<for> loop.
444 =head2 Basic BLOCKs and Switch Statements
446 A BLOCK by itself (labeled or not) is semantically equivalent to a
447 loop that executes once. Thus you can use any of the loop control
448 statements in it to leave or restart the block. (Note that this is
449 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
450 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
453 The BLOCK construct is particularly nice for doing case
457 if (/^abc/) { $abc = 1; last SWITCH; }
458 if (/^def/) { $def = 1; last SWITCH; }
459 if (/^xyz/) { $xyz = 1; last SWITCH; }
463 There is no official C<switch> statement in Perl, because there are
464 already several ways to write the equivalent.
466 However, starting from Perl 5.8 to get switch and case one can use
467 the Switch extension and say:
471 after which one has switch and case. It is not as fast as it could be
472 because it's not really part of the language (it's done using source
473 filters) but it is available, and it's very flexible.
475 In addition to the above BLOCK construct, you could write
478 $abc = 1, last SWITCH if /^abc/;
479 $def = 1, last SWITCH if /^def/;
480 $xyz = 1, last SWITCH if /^xyz/;
484 (That's actually not as strange as it looks once you realize that you can
485 use loop control "operators" within an expression. That's just the binary
486 comma operator in scalar context. See L<perlop/"Comma Operator">.)
491 /^abc/ && do { $abc = 1; last SWITCH; };
492 /^def/ && do { $def = 1; last SWITCH; };
493 /^xyz/ && do { $xyz = 1; last SWITCH; };
497 or formatted so it stands out more as a "proper" C<switch> statement:
520 /^abc/ and $abc = 1, last SWITCH;
521 /^def/ and $def = 1, last SWITCH;
522 /^xyz/ and $xyz = 1, last SWITCH;
537 A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
538 a temporary assignment to C<$_> for convenient matching:
540 SWITCH: for ($where) {
541 /In Card Names/ && do { push @flags, '-e'; last; };
542 /Anywhere/ && do { push @flags, '-h'; last; };
543 /In Rulings/ && do { last; };
544 die "unknown value for form variable where: `$where'";
547 Another interesting approach to a switch statement is arrange
548 for a C<do> block to return the proper value:
551 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
552 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
553 elsif ($flag & O_RDWR) {
554 if ($flag & O_CREAT) { "w+" }
555 else { ($flag & O_APPEND) ? "a+" : "r+" }
562 ($flags & O_WRONLY) ? "write-only" :
563 ($flags & O_RDWR) ? "read-write" :
567 Or if you are certain that all the C<&&> clauses are true, you can use
568 something like this, which "switches" on the value of the
569 C<HTTP_USER_AGENT> environment variable.
572 # pick out jargon file page based on browser
573 $dir = 'http://www.wins.uva.nl/~mes/jargon';
574 for ($ENV{HTTP_USER_AGENT}) {
575 $page = /Mac/ && 'm/Macintrash.html'
576 || /Win(dows )?NT/ && 'e/evilandrude.html'
577 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
578 || /Linux/ && 'l/Linux.html'
579 || /HP-UX/ && 'h/HP-SUX.html'
580 || /SunOS/ && 's/ScumOS.html'
581 || 'a/AppendixB.html';
583 print "Location: $dir/$page\015\012\015\012";
585 That kind of switch statement only works when you know the C<&&> clauses
586 will be true. If you don't, the previous C<?:> example should be used.
588 You might also consider writing a hash of subroutine references
589 instead of synthesizing a C<switch> statement.
593 Although not for the faint of heart, Perl does support a C<goto>
594 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
595 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
596 a C<goto>; it's just the name of the loop.
598 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
599 execution there. It may not be used to go into any construct that
600 requires initialization, such as a subroutine or a C<foreach> loop. It
601 also can't be used to go into a construct that is optimized away. It
602 can be used to go almost anywhere else within the dynamic scope,
603 including out of subroutines, but it's usually better to use some other
604 construct such as C<last> or C<die>. The author of Perl has never felt the
605 need to use this form of C<goto> (in Perl, that is--C is another matter).
607 The C<goto>-EXPR form expects a label name, whose scope will be resolved
608 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
609 necessarily recommended if you're optimizing for maintainability:
611 goto(("FOO", "BAR", "GLARCH")[$i]);
613 The C<goto>-&NAME form is highly magical, and substitutes a call to the
614 named subroutine for the currently running subroutine. This is used by
615 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
616 pretend that the other subroutine had been called in the first place
617 (except that any modifications to C<@_> in the current subroutine are
618 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
619 will be able to tell that this routine was called first.
621 In almost all cases like this, it's usually a far, far better idea to use the
622 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
623 resorting to a C<goto>. For certain applications, the catch and throw pair of
624 C<eval{}> and die() for exception processing can also be a prudent approach.
626 =head2 PODs: Embedded Documentation
628 Perl has a mechanism for intermixing documentation with source code.
629 While it's expecting the beginning of a new statement, if the compiler
630 encounters a line that begins with an equal sign and a word, like this
632 =head1 Here There Be Pods!
634 Then that text and all remaining text up through and including a line
635 beginning with C<=cut> will be ignored. The format of the intervening
636 text is described in L<perlpod>.
638 This allows you to intermix your source code
639 and your documentation text freely, as in
643 The snazzle() function will behave in the most spectacular
644 form that you can possibly imagine, not even excepting
645 cybernetic pyrotechnics.
647 =cut back to the compiler, nuff of this pod stuff!
654 Note that pod translators should look at only paragraphs beginning
655 with a pod directive (it makes parsing easier), whereas the compiler
656 actually knows to look for pod escapes even in the middle of a
657 paragraph. This means that the following secret stuff will be
658 ignored by both the compiler and the translators.
662 warn "Neither POD nor CODE!?"
666 You probably shouldn't rely upon the C<warn()> being podded out forever.
667 Not all pod translators are well-behaved in this regard, and perhaps
668 the compiler will become pickier.
670 One may also use pod directives to quickly comment out a section
673 =head2 Plain Old Comments (Not!)
675 Perl can process line directives, much like the C preprocessor. Using
676 this, one can control Perl's idea of filenames and line numbers in
677 error or warning messages (especially for strings that are processed
678 with C<eval()>). The syntax for this mechanism is the same as for most
679 C preprocessors: it matches the regular expression
681 # example: '# line 42 "new_filename.plx"'
684 (?:\s("?)([^"]+)\2)? \s*
687 with C<$1> being the line number for the next line, and C<$3> being
688 the optional filename (specified with or without quotes).
690 There is a fairly obvious gotcha included with the line directive:
691 Debuggers and profilers will only show the last source line to appear
692 at a particular line number in a given file. Care should be taken not
693 to cause line number collisions in code you'd like to debug later.
695 Here are some examples that you should be able to type into your command
700 # the `#' on the previous line must be the first char on line
703 foo at bzzzt line 201.
707 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
712 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
714 foo at foo bar line 200.
718 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
721 foo at goop line 345.