7 A Perl script consists of a sequence of declarations and statements.
8 The only things that need to be declared in Perl are report formats
9 and subroutines. See the sections below for more information on those
10 declarations. All uninitialized user-created objects are assumed to
11 start with a C<null> or C<0> value until they are defined by some explicit
12 operation such as assignment. (Though you can get warnings about the
13 use of undefined values if you like.) The sequence of statements is
14 executed just once, unlike in B<sed> and B<awk> scripts, where the
15 sequence of statements is executed for each input line. While this means
16 that you must explicitly loop over the lines of your input file (or
17 files), it also means you have much more control over which files and
18 which lines you look at. (Actually, I'm lying--it is possible to do an
19 implicit loop with either the B<-n> or B<-p> switch. It's just not the
20 mandatory default like it is in B<sed> and B<awk>.)
24 Perl is, for the most part, a free-form language. (The only exception
25 to this is format declarations, for obvious reasons.) Text from a
26 C<"#"> character until the end of the line is a comment, and is
27 ignored. If you attempt to use C</* */> C-style comments, it will be
28 interpreted either as division or pattern matching, depending on the
29 context, and C++ C<//> comments just look like a null regular
30 expression, so don't do that.
32 A declaration can be put anywhere a statement can, but has no effect on
33 the execution of the primary sequence of statements--declarations all
34 take effect at compile time. Typically all the declarations are put at
35 the beginning or the end of the script. However, if you're using
36 lexically-scoped private variables created with C<my()>, you'll have to make sure
37 your format or subroutine definition is within the same block scope
38 as the my if you expect to be able to access those private variables.
40 Declaring a subroutine allows a subroutine name to be used as if it were a
41 list operator from that point forward in the program. You can declare a
42 subroutine without defining it by saying C<sub name>, thus:
45 $me = myname $0 or die "can't get myname";
47 Note that my() functions as a list operator, not as a unary operator; so
48 be careful to use C<or> instead of C<||> in this case. However, if
49 you were to declare the subroutine as C<sub myname ($)>, then
50 C<myname> would function as a unary operator, so either C<or> or
53 Subroutines declarations can also be loaded up with the C<require> statement
54 or both loaded and imported into your namespace with a C<use> statement.
55 See L<perlmod> for details on this.
57 A statement sequence may contain declarations of lexically-scoped
58 variables, but apart from declaring a variable name, the declaration acts
59 like an ordinary statement, and is elaborated within the sequence of
60 statements as if it were an ordinary statement. That means it actually
61 has both compile-time and run-time effects.
63 =head2 Simple statements
65 The only kind of simple statement is an expression evaluated for its
66 side effects. Every simple statement must be terminated with a
67 semicolon, unless it is the final statement in a block, in which case
68 the semicolon is optional. (A semicolon is still encouraged there if the
69 block takes up more than one line, because you may eventually add another line.)
70 Note that there are some operators like C<eval {}> and C<do {}> that look
71 like compound statements, but aren't (they're just TERMs in an expression),
72 and thus need an explicit termination if used as the last item in a statement.
74 Any simple statement may optionally be followed by a I<SINGLE> modifier,
75 just before the terminating semicolon (or block ending). The possible
84 The C<if> and C<unless> modifiers have the expected semantics,
85 presuming you're a speaker of English. The C<foreach> modifier is an
86 iterator: For each value in EXPR, it aliases C<$_> to the value and
87 executes the statement. The C<while> and C<until> modifiers have the
88 usual "C<while> loop" semantics (conditional evaluated first), except
89 when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE
90 statement), in which case the block executes once before the
91 conditional is evaluated. This is so that you can write loops like:
96 } until $line eq ".\n";
98 See L<perlfunc/do>. Note also that the loop control statements described
99 later will I<NOT> work in this construct, because modifiers don't take
100 loop labels. Sorry. You can always put another block inside of it
101 (for C<next>) or around it (for C<last>) to do that sort of thing.
102 For C<next>, just double the braces:
109 For C<last>, you have to be more elaborate:
118 =head2 Compound statements
120 In Perl, a sequence of statements that defines a scope is called a block.
121 Sometimes a block is delimited by the file containing it (in the case
122 of a required file, or the program as a whole), and sometimes a block
123 is delimited by the extent of a string (in the case of an eval).
125 But generally, a block is delimited by curly brackets, also known as braces.
126 We will call this syntactic construct a BLOCK.
128 The following compound statements may be used to control flow:
131 if (EXPR) BLOCK else BLOCK
132 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
133 LABEL while (EXPR) BLOCK
134 LABEL while (EXPR) BLOCK continue BLOCK
135 LABEL for (EXPR; EXPR; EXPR) BLOCK
136 LABEL foreach VAR (LIST) BLOCK
137 LABEL BLOCK continue BLOCK
139 Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
140 not statements. This means that the curly brackets are I<required>--no
141 dangling statements allowed. If you want to write conditionals without
142 curly brackets there are several other ways to do it. The following
143 all do the same thing:
145 if (!open(FOO)) { die "Can't open $FOO: $!"; }
146 die "Can't open $FOO: $!" unless open(FOO);
147 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
148 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
149 # a bit exotic, that last one
151 The C<if> statement is straightforward. Because BLOCKs are always
152 bounded by curly brackets, there is never any ambiguity about which
153 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
154 the sense of the test is reversed.
156 The C<while> statement executes the block as long as the expression is
157 true (does not evaluate to the null string (C<"">) or C<0> or C<"0")>. The LABEL is
158 optional, and if present, consists of an identifier followed by a colon.
159 The LABEL identifies the loop for the loop control statements C<next>,
160 C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
161 refers to the innermost enclosing loop. This may include dynamically
162 looking back your call-stack at run time to find the LABEL. Such
163 desperate behavior triggers a warning if you use the B<-w> flag.
165 If there is a C<continue> BLOCK, it is always executed just before the
166 conditional is about to be evaluated again, just like the third part of a
167 C<for> loop in C. Thus it can be used to increment a loop variable, even
168 when the loop has been continued via the C<next> statement (which is
169 similar to the C C<continue> statement).
173 The C<next> command is like the C<continue> statement in C; it starts
174 the next iteration of the loop:
176 LINE: while (<STDIN>) {
177 next LINE if /^#/; # discard comments
181 The C<last> command is like the C<break> statement in C (as used in
182 loops); it immediately exits the loop in question. The
183 C<continue> block, if any, is not executed:
185 LINE: while (<STDIN>) {
186 last LINE if /^$/; # exit when done with header
190 The C<redo> command restarts the loop block without evaluating the
191 conditional again. The C<continue> block, if any, is I<not> executed.
192 This command is normally used by programs that want to lie to themselves
193 about what was just input.
195 For example, when processing a file like F</etc/termcap>.
196 If your input lines might end in backslashes to indicate continuation, you
197 want to skip ahead and get the next record.
208 which is Perl short-hand for the more explicitly written version:
210 LINE: while (defined($line = <ARGV>)) {
212 if ($line =~ s/\\$//) {
214 redo LINE unless eof(); # not eof(ARGV)!
219 Note that if there were a C<continue> block on the above code, it would get
220 executed even on discarded lines. This is often used to reset line counters
221 or C<?pat?> one-time matches.
223 # inspired by :1,$g/fred/s//WILMA/
225 ?(fred)? && s//WILMA $1 WILMA/;
226 ?(barney)? && s//BETTY $1 BETTY/;
227 ?(homer)? && s//MARGE $1 MARGE/;
229 print "$ARGV $.: $_";
230 close ARGV if eof(); # reset $.
231 reset if eof(); # reset ?pat?
234 If the word C<while> is replaced by the word C<until>, the sense of the
235 test is reversed, but the conditional is still tested before the first
238 The loop control statements don't work in an C<if> or C<unless>, since
239 they aren't loops. You can double the braces to make them such, though.
247 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
248 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
252 Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
253 that means that this:
255 for ($i = 1; $i < 10; $i++) {
268 (There is one minor difference: The first form implies a lexical scope
269 for variables declared with C<my> in the initialization expression.)
271 Besides the normal array index looping, C<for> can lend itself
272 to many other interesting applications. Here's one that avoids the
273 problem you get into if you explicitly test for end-of-file on
274 an interactive file descriptor causing your program to appear to
277 $on_a_tty = -t STDIN && -t STDOUT;
278 sub prompt { print "yes? " if $on_a_tty }
279 for ( prompt(); <STDIN>; prompt() ) {
285 The C<foreach> loop iterates over a normal list value and sets the
286 variable VAR to be each element of the list in turn. If the variable
287 is preceded with the keyword C<my>, then it is lexically scoped, and
288 is therefore visible only within the loop. Otherwise, the variable is
289 implicitly local to the loop and regains its former value upon exiting
290 the loop. If the variable was previously declared with C<my>, it uses
291 that variable instead of the global one, but it's still localized to
294 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
295 you can use C<foreach> for readability or C<for> for brevity. (Or because
296 the Bourne shell is more familiar to you than I<csh>, so writing C<for>
297 comes more naturally.) If VAR is omitted, C<$_> is set to each value.
298 If any element of LIST is an lvalue, you can modify it by modifying VAR
299 inside the loop. That's because the C<foreach> loop index variable is
300 an implicit alias for each item in the list that you're looping over.
302 If any part of LIST is an array, C<foreach> will get very confused if
303 you add or remove elements within the loop body, for example with
304 C<splice>. So don't do that.
306 C<foreach> probably won't do what you expect if VAR is a tied or other
307 special variable. Don't do that either.
311 for (@ary) { s/foo/bar/ }
313 foreach my $elem (@elements) {
317 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
318 print $count, "\n"; sleep(1);
321 for (1..15) { print "Merry Christmas\n"; }
323 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
324 print "Item: $item\n";
327 Here's how a C programmer might code up a particular algorithm in Perl:
329 for (my $i = 0; $i < @ary1; $i++) {
330 for (my $j = 0; $j < @ary2; $j++) {
331 if ($ary1[$i] > $ary2[$j]) {
332 last; # can't go to outer :-(
334 $ary1[$i] += $ary2[$j];
336 # this is where that last takes me
339 Whereas here's how a Perl programmer more comfortable with the idiom might
342 OUTER: foreach my $wid (@ary1) {
343 INNER: foreach my $jet (@ary2) {
344 next OUTER if $wid > $jet;
349 See how much easier this is? It's cleaner, safer, and faster. It's
350 cleaner because it's less noisy. It's safer because if code gets added
351 between the inner and outer loops later on, the new code won't be
352 accidentally executed. The C<next> explicitly iterates the other loop
353 rather than merely terminating the inner one. And it's faster because
354 Perl executes a C<foreach> statement more rapidly than it would the
355 equivalent C<for> loop.
357 =head2 Basic BLOCKs and Switch Statements
359 A BLOCK by itself (labeled or not) is semantically equivalent to a
360 loop that executes once. Thus you can use any of the loop control
361 statements in it to leave or restart the block. (Note that this is
362 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
363 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
366 The BLOCK construct is particularly nice for doing case
370 if (/^abc/) { $abc = 1; last SWITCH; }
371 if (/^def/) { $def = 1; last SWITCH; }
372 if (/^xyz/) { $xyz = 1; last SWITCH; }
376 There is no official C<switch> statement in Perl, because there are
377 already several ways to write the equivalent. In addition to the
378 above, you could write
381 $abc = 1, last SWITCH if /^abc/;
382 $def = 1, last SWITCH if /^def/;
383 $xyz = 1, last SWITCH if /^xyz/;
387 (That's actually not as strange as it looks once you realize that you can
388 use loop control "operators" within an expression, That's just the normal
394 /^abc/ && do { $abc = 1; last SWITCH; };
395 /^def/ && do { $def = 1; last SWITCH; };
396 /^xyz/ && do { $xyz = 1; last SWITCH; };
400 or formatted so it stands out more as a "proper" C<switch> statement:
423 /^abc/ and $abc = 1, last SWITCH;
424 /^def/ and $def = 1, last SWITCH;
425 /^xyz/ and $xyz = 1, last SWITCH;
440 A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
441 a temporary assignment to C<$_> for convenient matching:
443 SWITCH: for ($where) {
444 /In Card Names/ && do { push @flags, '-e'; last; };
445 /Anywhere/ && do { push @flags, '-h'; last; };
446 /In Rulings/ && do { last; };
447 die "unknown value for form variable where: `$where'";
450 Another interesting approach to a switch statement is arrange
451 for a C<do> block to return the proper value:
454 if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
455 elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
456 elsif ($flag & O_RDWR) {
457 if ($flag & O_CREAT) { "w+" }
458 else { ($flag & O_APPEND) ? "a+" : "r+" }
465 ($flags & O_WRONLY) ? "write-only" :
466 ($flags & O_RDWR) ? "read-write" :
470 Or if you are certainly that all the C<&&> clauses are true, you can use
471 something like this, which "switches" on the value of the
472 C<HTTP_USER_AGENT> envariable.
475 # pick out jargon file page based on browser
476 $dir = 'http://www.wins.uva.nl/~mes/jargon';
477 for ($ENV{HTTP_USER_AGENT}) {
478 $page = /Mac/ && 'm/Macintrash.html'
479 || /Win(dows )?NT/ && 'e/evilandrude.html'
480 || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
481 || /Linux/ && 'l/Linux.html'
482 || /HP-UX/ && 'h/HP-SUX.html'
483 || /SunOS/ && 's/ScumOS.html'
484 || 'a/AppendixB.html';
486 print "Location: $dir/$page\015\012\015\012";
488 That kind of switch statement only works when you know the C<&&> clauses
489 will be true. If you don't, the previous C<?:> example should be used.
491 You might also consider writing a hash of subroutine references
492 instead of synthesizing a C<switch> statement.
496 Although not for the faint of heart, Perl does support a C<goto>
497 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
498 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
499 a C<goto>; it's just the name of the loop.
501 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
502 execution there. It may not be used to go into any construct that
503 requires initialization, such as a subroutine or a C<foreach> loop. It
504 also can't be used to go into a construct that is optimized away. It
505 can be used to go almost anywhere else within the dynamic scope,
506 including out of subroutines, but it's usually better to use some other
507 construct such as C<last> or C<die>. The author of Perl has never felt the
508 need to use this form of C<goto> (in Perl, that is--C is another matter).
510 The C<goto>-EXPR form expects a label name, whose scope will be resolved
511 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
512 necessarily recommended if you're optimizing for maintainability:
514 goto ("FOO", "BAR", "GLARCH")[$i];
516 The C<goto>-&NAME form is highly magical, and substitutes a call to the
517 named subroutine for the currently running subroutine. This is used by
518 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
519 pretend that the other subroutine had been called in the first place
520 (except that any modifications to C<@_> in the current subroutine are
521 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
522 will be able to tell that this routine was called first.
524 In almost all cases like this, it's usually a far, far better idea to use the
525 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
526 resorting to a C<goto>. For certain applications, the catch and throw pair of
527 C<eval{}> and die() for exception processing can also be a prudent approach.
529 =head2 PODs: Embedded Documentation
531 Perl has a mechanism for intermixing documentation with source code.
532 While it's expecting the beginning of a new statement, if the compiler
533 encounters a line that begins with an equal sign and a word, like this
535 =head1 Here There Be Pods!
537 Then that text and all remaining text up through and including a line
538 beginning with C<=cut> will be ignored. The format of the intervening
539 text is described in L<perlpod>.
541 This allows you to intermix your source code
542 and your documentation text freely, as in
546 The snazzle() function will behave in the most spectacular
547 form that you can possibly imagine, not even excepting
548 cybernetic pyrotechnics.
550 =cut back to the compiler, nuff of this pod stuff!
557 Note that pod translators should look at only paragraphs beginning
558 with a pod directive (it makes parsing easier), whereas the compiler
559 actually knows to look for pod escapes even in the middle of a
560 paragraph. This means that the following secret stuff will be
561 ignored by both the compiler and the translators.
565 warn "Neither POD nor CODE!?"
569 You probably shouldn't rely upon the C<warn()> being podded out forever.
570 Not all pod translators are well-behaved in this regard, and perhaps
571 the compiler will become pickier.
573 One may also use pod directives to quickly comment out a section
576 =head2 Plain Old Comments (Not!)
578 Much like the C preprocessor, Perl can process line directives. Using
579 this, one can control Perl's idea of filenames and line numbers in
580 error or warning messages (especially for strings that are processed
581 with C<eval()>). The syntax for this mechanism is the same as for most
582 C preprocessors: it matches the regular expression
583 C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
584 number for the next line, and C<$2> being the optional filename
585 (specified within quotes).
587 Here are some examples that you should be able to type into your command
592 # the `#' on the previous line must be the first char on line
595 foo at bzzzt line 201.
599 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
604 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
606 foo at foo bar line 200.
610 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
613 foo at goop line 345.