Hm. :encoding() not being a function, and perldoc encoding talking about something unrelated, it wasn't immediately clear where to find :encoding()'s documentation. I ended up choosing utf8 over UTF-8 because perldoc Encode::Supported, which I found when casting about for encoding info, indicated the former form was canonical. But now I suspect I should have been looking at perldoc Encode instead.
But I'm still not completely sure. Perl's encoding documentation is a twisty maze of little passages, all different, and I've not yet found a signpost definitively stating "These are the legal arguments to :encoding() and what they mean."