Excellent points++! It's evident, however, that the OP's considering the possibility of characters repeating in strings, as shown in the initial posting:

my ($str1, $str2) = ("ABCDE", "BBBBB");

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In reply to Re^2: compare two strings and return only he unique values. by Kenosis
in thread compare two strings and return only he unique values. by Learnlot1

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