I'd expect both of the following calls to output the same thing, so I think it's a bug.

use utf8;
sub ff {
  print utf8::is_utf8($_[0]) ? 1 : 0, "\n";
}
ff(asd => 1);  # 1
ff("asd");     # 0
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Tested in 5.8.8 and 5.10.0.