$ perl -CS -le 'print "na\x{EF}vet\x{E9}"' |
  perl -C0 -pe 's/(\P{ASCII})/"&#".ord($1).";"/ge' | 
  perl -MEncode -C0 -pe 's/&#(\d+);/chr($1)/ge;
                                    $_ = decode_utf8($_, 1); 
                                    s/(\P{ASCII})/"&#".ord($1).";"/ge'
naïveté