my $utf8_decodable_regex =
    qr/[\xC0-\xDF][\x80-\xBF]    | # 2 bytes unicode char
       [\xE0-\xEF][\x80-\xBF]{2} | # 3 bytes unicode char
       [\xF0-\xFF][\x80-\xBF]{3}/x;

    $testStr = decode('utf-8',$testStr);
    $testStr =~ s/($utf8_decodable_regex)/decode('utf-8',$1)/gex;
    $testStr = encode('utf-8',$testStr);