in reply to Unique List of Common Characters Between Two Strings

Or, if you're a regex fan, you might like... 
sub In_Common2 
{
     my ($str1, $str2) = @_;
    (my $common = reverse $str2) =~ s/[^$str1]|(.)(?=.*\1)//g;

     return scalar reverse $common;
}

[download]

-- All code is 100% tested and functional unless otherwise noted.

In Section Cool Uses for Perl