After some investigation, barewords in general (anything parsed by force_word) are affected.

use utf8;
sub ff { print utf8::is_utf8($_[0]) ? 1 : 0, "\n"; }

ff("asd");               # 0
ff('asd');               # 0
ff(qw(asd));             # 0

ff(asd => 1);            # 1
{ no strict; ff(asd); }  # 1
ff(-asd);                # 1
ff(asd::);               # 1
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I think function names (including built-ins) and keywords are also affected.

Gotta go. No time to devise a fix atm.