OK, the culprit is:
Removing that line, or changing to utf8 (the non-strict sister encoding to :encoding(UTF-8)) fixes the warning. That said, I have no idea what the root cause is or how to actually fix this.
@tokuhirom, @schwern: I can silence the warning by simply switching the encoding set on that line, but I don't think that is a good solution. I prefer to use utf-8-strict because it picks up this sort of thing, but more importantly, I'm not sure why Perl is issuing that warning, since the character occurs outside anything we should be touching. Simply silencing the warning without understanding why it is issued makes me feel icky. Unfortunately, I don't think I know enough to fix this properly without help.
@tokuhirom, @schwern: see 221e994 for my sort-of-proposed fix.
As said here, the issue is with perl itself, not with utf8::all. Changing from :encoding(utf8) to :utf8 will kill the warning but not the real problem. I don't know yet if it's possible to work around this problem though.
This was discussed at some point on p5p. I meant to ask if there was a workaround older Perls could use, but I don't think I ever did. I should check who was talking about this and get in touch with them about it.
This is a bug in Perl.