in reply to Replacing letters in one string based on another

$str1 = "ABCXXFGH"; $str2 = "XXCDEFGH";;

tr[X][\x00] for $str1, $str2;;

( $str3 = $str1 & $str2 ) =~ tr[\x00][X];;

print $str3;;
XXCXXFGH

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In Section Seekers of Perl Wisdom