Interesting point. That's not what I did though. My solution was

$ref->{ utf8_on($key) } = $ref->{$key};
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and yet, I did not double the size of the hash, as proven by tests I have written to count and enumerate the keys as received in Python from Perl. I believe that my code "works" without duplicating the keys because utf8_on is not changing or manipulating the actual string that is the key; all it is doing is manipulating the metadata.