In 5.10 IIRC we were no longer able to say 'please leave it alone', we had to say 'please temporarily treat it as a string of bytes, and then make me a copy that doesn't have the utf8 flag'. The semantics of encode_utf8 are something closer to 'make me a copy in Perl's utf8 encoding and return it as a sequence of bytes without the utf8 flag', which has the same result without being a kludge. I guess you could test it for a bunch of random byte sequences if you had to. The only differences I see are that we won't be making copies anymore when the original string doesn't have the utf8 flag, and we dropped compatibility for Perl < 5.8. Shrug.

That said, I think you missed the part where sophie wants us to stop managing it ourselves and actually use characters.