The easiest thing to do is to create a hash of code refs. Pull out the query string or URL information or whatever, then use that data as the hash key.

If there's a valid code ref there, call that subroutine. If not, show an error or do a default sub.

For example, you could write:

my %sub_refs = (
    foo => \&do_this,
    bar => \&do_that,
);

my $request = $ENV{QUERY_STRING};

if (defined(my $action = $sub_refs{$request})) {
    $action->();
} else {
    default();
}
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