If you follow the links above to the perldoc pack page, you'll find this description:

If the pattern begins with a U , the resulting string will be treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string with an initial U0 , and the bytes that follow will be interpreted as Unicode characters. If you don't want this to happen, you can begin your pattern with C0 (or anything else) to force Perl not to UTF-8 encode your string, and then follow this with a U* somewhere in your pattern.