sub is_prime {
    my %prime = map { $_ => 1 } (2, 3, 5, 7);
    my %not_prime;
    return sub {
        my $n = shift;
        return 1 if $prime{ $n };
        return 0 if $n % 2 == 0 || $n % 3 == 0 
                 || exists $not_prime{ $n };
        my $last = int sqrt $n;
        for ( my $x = 5; $x <= $last ;) {
            return $not_prime{ $n } = 0 if $n % $x == 0;
            $x += 2;
            return $not_prime{ $n } = 0 if $n % $x == 0;
            $x += 4;
        }
        return $prime{ $n } = 1;
    };
}