Programming Perl, 4th Edition

The given Statement

In the previous example, we kept talking about $color. Linguists call this a topic. In v5.10 or later, an alternative to the if structure is available, the given statement, which functions linguistically as a topicalizer. It works by setting $_ to the current topic. You can then use when statements to examine the topic for various values or patterns.

This feature is enabled when you use a version of Perl that is at least v5.10:

use v5.12;              # at least v5.12, load default features

or when you specifically request the “switch” feature:

use feature "switch";   # just get the switch feature

Either of those adds several new keywords to the Perl language: given, when, break, continue, and default. Here is one way to recode the previous example using the new feature:

use v5.10;

my $value;
given (<STDIN>) {
    when (/red/i)    { $value = 0xFF0000 }
    when (/green/i)  { $value = 0x00FF00 }
    when (/blue/i)   { $value = 0x0000FF; }
    default {
        chomp;
        warn "unknown RGB component '$_', using black instead\n";
        $value = 0x000000;
    }
}

In fact, in v5.10 you had to write it that way since given couldn’t return values. In v5.14 or later, you can return values, and with the statement modifier form of when, you may even write it this way:

use v5.14;

my $value = do {
    given (<STDIN>) {
        0xFF0000 when /red/i;
        0x00FF00 when /green/i;
        0x0000FF when /blue/i;
        chomp;
        warn "unknown RGB component '$_', using black instead\n";
        0x000000;
    }
};

The arguments to given and when are in scalar context; given binds its argument ...