sub foo ($) {
    my $arg = shift;
    state %cache;

    if exists $cache{$arg} { 
       print "Got that from the cache!\n";
       return $cache{$arg} 
    }
    print "Didn't find that in cache...\n";

    my $result = ... something costly ...;
    $cache{$arg} = $result;
    return $result;
}