sub is_prime {
    my $guess = shift;
    # accidentally reversed next two lines. Thanks to petral for catching that :)
    return 1 if $guess == 2;
    return unless $guess > 2 and $guess % 2;

    my $max = 1 + int $guess ** .5;
    for ( my $divisor = 3; $divisor < $max; $divisor += 2 ) {
        return unless $guess % $divisor;
    }
    return 1;
}