Programming Perl, 4th Edition

Loopy Topicalizers

Perl has more than one topicalizer; in addition to given, you can also use a foreach loop as a topicalizer. For example, here’s one way to count how many times a particular string occurs in an array:

use v5.10.1;
my $count = 0;
for (@array) {
    when ("FNORD") { ++$count }
}
print "\@array contains $count copies of 'FNORD'\n";

Or in a more recent version:

use v5.14;
my $count = 0;
for (@array) {
    ++$count when "FNORD";
}
print "\@array contains $count copies of 'FNORD'\n";

At the end of all when blocks inside a foreach loop, there is an implicit break, which, since you’re in a loop, is equivalent to a next. You can override that with an explicit last if you’re only interested in the first match.

A when only works if the topic is in $_, so you can’t specify a loop variable, or if you do, it must be $_:

for my $_ (@answers) {
    say "Life, the Universe, and Everything!" when 42;
}