Programming Perl, 4th Edition

The given and when Statements

To test a single value for a bunch of different alternatives, recent versions of Perl have what other languages sometimes call switch and case. Because we like to make Perl work like a natural language, however, we call these given and when. (Since you’re already putting use v5.14 at the top, you should have this functionality, which was introduced in 5.10.)

#!/usr/bin/perl
use v5.14;

print "What is your favorite color? ";
chomp(my $answer = <STDIN>);

given ($answer) {
    when ("purple")      { say "Me too." }

    when ("green")       { say "Go!" }
    when ("yellow")      { say "Slow!" }
    when ("red")         { say "Stop!" }

    when ("blue")        { say "You may proceed." }
    when (/\w+, no \w+/) { die "AAAUUUGHHHHH!" }

    when (42)            { say "Wrong answer." }

    when (['gray','orange','brown','black','white']) {
        say "I think $answer is pretty okay too.";
    }

    default {
        say "Are you sure $answer is a real color?";
    }
}

First the given part takes the value of its expression and makes it the topic of conversation, so the when statements know which value to test. The cases are then evaluated by matching the argument of each when against the topic to find the first when statement that thinks the topic’s value matches. The when statements try to match in order, and as soon as one matches, it doesn’t try any of the subsequent statements, but drops out of the whole given construct.

The form of each when argument ("red" vs 42 vs /\w+, no \w+/) determines the type of match performed, so strings match as strings, numbers ...