It seems I did not truly realize that perl has an internal way of using UTF-8 that has nothing to do with what encoding the shell is using.

Perl's internal way of representing unicode characters is (almost) UTF-8, too.  But for most practical purposes from the user perspective, it helps to ignore this implementation detail¹, and just properly decode your inputs and encode your outputs.

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¹ other languages have chosen different internal formats for unicode strings, e.g. Python uses UCS-2 or UCS-4 (build-time option).