encoding produces the opposite result.

#use encoding 'UTF-8';  # or 'utf8'
#use utf8;

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

                         # none enco utf8 both
                         # ---- ---- ---- ----
ff("asd");               # 0    1    0    1
ff('asd');               # 0    1    0    1
ff(qw(asd));             # 0    1    0    1

ff(asd => 1);            # 0    0    1    1
{ no strict; ff(asd); }  # 0    0    1    1
ff(-asd);                # 0    0    1    1
ff(asd::);               # 0    0    1    1
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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.

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.