#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