I think you might be right about that entry in perl5220delta. Just for reference, I think that the corresponding commit is f13c8ddbf, which references this P5P thread.

Of course, we really need tothestars to show a SSCCE, then it'd even be possible to run a git blame to narrow it down to the exact commit. But using the above information, it might be something related to misdetection of UTF-8.

tothestars: Note that -T is a heuristic test only, for example, the documentation has always said (emphasis mine): "The first block or so of the file is examined". If you want any kind of certainty, you should use a more reliable method - what's best here depends on what you're trying to achieve with this test.