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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In reply to Re^2: Unexpected utf8 in hash keys (encoding) by ikegami
in thread Unexpected utf8 in hash keys by kappa

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