STDOUT isn't really a file handle in your case. It's a tied object that presents the interface of a handle, but isn't actually. And layers (such as :encoding(UTF-8)) aren't supported by a tied handles.

I rather suspect that a similar problem could lurk in incoming data as well. I would certainly check if incoming Umlauts, Emojis and other Unicode stuff gets decoded correctly. In the long run, it might also pay to run some Unicode normalization to make sure the same text is always encoded the same way (especially for usernames, passwords and such). Unicode equivalence can be rather annoying sometimes, see also: incorrect length of strings with diphthongs.