in reply to
Unexpected utf8 in hash keys
I may be reading this too quickly and not appreciating or understanding your intent, or what exactly you are using the use utf8 pragma for or why.
My hunch is that you might benefit from not enquiring as to when the utf8 flag is set or not. Can you tell me your purposes?
We can play around with the internals of Perl and how it represents strings all day long, but I gather you have more than academic interest here?
This may be a place where solid pragmatism trumps.
Have you read perlunitut?