$_= "this is a TEsT case";

           $old = 'test';
           $new = 'success';

           s{(\Q$old\E)}
            { uc $new | (uc $1 ^ $1) .
               (uc(substr $1, -1) ^ substr $1, -1) x
                   (length($new) - length $1)
            }egi;

           print;