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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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4.6 Compound Statements

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

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

The following compound statements may be used to control flow:

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

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

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

The if statement is straightforward. Because BLOCKs are always bounded by curly brackets, there is never any ambiguity about which if an else goes with. If you use unless in place of if, the sense of the test is reversed. Like if, unless can be followed by else. unless can even be followed by one or more elsif statements, though you may want to think twice before using that particular language construct, as everyone reading your code will have to think at least twice before they can understand what's going on.

The while statement executes the block as long as the expression is true ( 4.4). The until statement executes the block as long as the expression is false. The LABEL is optional, and if present, consists of an identifier followed by a colon. The LABEL identifies the loop for the loop control statements next, last, and redo. If the LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include dynamically looking back your call-stack at run time to find the LABEL. Such desperate behavior triggers a warning if you use the use warnings pragma or the -w flag.

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

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

ISBN 9781906966027Perl Language Reference ManualSee the print edition