BEGIN {
    my @primes = (2, 3); my $max = 3;
    sub isPrime {
        my $num = shift;
        my $root = int sqrt $num;
        while ($max <= $root) {
             $max += 2;
             push(@primes, $max) if isPrime($max);
        }
        for (@primes) {
            last if $_ > $root;
            return 0 if ($num % $_ == 0);
        }
        return 1;
    }
}