Is that possible or already existing, or already planned?
Not for perl5, but feel free to submit a patch.
Am I missing something here?
Such a substitution is probably far less common that you might think. Not common enough to use a regex flag and increase the complexity of perl (the interpreter). It would
also start endless discussions of what the behaviour would
be if the replacement is longer/shorter than the original,
and cases where the match is in lower-case, the replacement is in upper-case, and there's no lower-case equivalent for
said upper-case. Not to mention the symantics of the /eI