in reply to Unique List of Common Characters Between Two Strings

YAWTDI: 
    my %u = map {$_ => $_} split //, $str1;
    return grep defined, delete @u{split //, $str2};

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Or, to bletch it all into one line: 
return grep defined, delete @{{map {$_ => $_} split //, $str1}}{split 
+//, $str2};

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Caution: Contents may have been coded under pressure.

In Section Cool Uses for Perl