use Encode;
my $s = get_s_from_somewhere();
my $chars = decode("UTF-8", $s);
my %non_ascii;

for my $i (0..length($chars)-1) {
  if( ord(substr($chars, $i, 1)) > 127 ) {
    $non_ascii{ substr($chars, $i, 1) }++;
  }
}

do_something_with_non_ascii(\%non_ascii)