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 combo.

Abigail