I doubt it. Check the output of perl -V.

Isn't PerlIO responsible for unicode?

Supporting unicode means changes in every part of perl. Regex, for exmaple.

Isn't there a way to say: don't expect encoding; go fast? If not, why not?

Do you have any idea how hard that is to do in a large and complex codebase like perl5? Parallel code paths for every operation? It's just not feasible.