my $_factorial;
$_factorial = sub {
    my ($result, $counter, $max) = @_;
    return $result if $counter > $max;
    @_ = (($result * $counter), ($counter + 1), $max);
    goto $_factorial;
};

sub factorial {
    my ($num) = @_;
    @_ = (1, 1, $num);
    goto $_factorial;
}