perldoc -f case

Here's the first part of the text from my docs:

How do I substitute case insensitively on the LHS while preserving case on the RHS?

Here's a lovely Perlish solution by Larry Rosler. It exploits properties of bitwise xor on ASCII strings.

           $_= "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;
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But you really should read the rest of the entry

update: I just realized that I may have slightly misread what you wanted to do. That's okay though as the technique (using the ^ operator) is basically the same. :-)