in reply to Unique List of Common Characters Between Two Strings

return grep { $u{ $_ } ? ( $u{ $_ }-- , $_ ) : () } split //, $str2;

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Better written as: 
return grep --$u{ $_ } >= 0, split //, $str2;

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-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

In Section Cool Uses for Perl