# A classic, recursive soulution.
    sub Rfactorial {
        my $i = shift;
        return 1 if $i == 0;
        return 1 if $i == 1;
        return $i * Rfactorial($i - 1);
    }

##</code><code>##

    # Tail recursive
    sub factorial {
        my $n = shift;
        return 0 unless $n > 0;
        fact( $n, 1 );
        
        sub fact {
            my($n, $a) = @_;
            return $a unless $n > 1;
            fact( $n-1, $n*$a );
        }
    }