This simplified my code a lot:

my $utf8 = q{ [\x00-\x7F] # One-byte range | [\xC2-\xDF][\x80-\xBF] # Two-byte range | \xE0[\xA0-\xBF][\x80-\xBF] # Three-byte range | [\xE1-\xEF][\x80-\xBF][\x80-\xBF] # Three-byte range | \xF0[\x90-\xBF][\x80-\xBF][\x80-\xBF] # Four-byte range | [\xF1-\xF7][\x80-\xBF][\x80-\xBF][\x80-\xBF] # Four-byte range | \xF8[\x88-\xBF][\x80-\xBF][\x80-\xBF][\x80-\xBF] # Five-byte range | [\xF9-\xFB][\x80-\xBF][\x80-\xBF][\x80-\xBF][\x80-\xBF] # Five-byte range | \xFC[\x84-\xBF][\x80-\xBF][\x80-\xBF][\x80-\xBF][\x80-\xBF] # Six-byte range | \xFD[\x80-\xBF][\x80-\xBF][\x80-\xBF][\x80-\xBF][\x80-\xBF] # Six-byte range };

Thanks, O'Reilly (PDF).