Messing with Perl's internals

    The following API uses parts of Perl's internals in the
    current implementation.  As such, they are efficient but
    may change.

    is_utf8(STRING [, CHECK])
      [INTERNAL] Tests whether the UTF-8 flag is turned on
      in the STRING.  If CHECK is true, also checks the data
      in STRING for being well-formed UTF-8.  Returns true
      if successful, false otherwise.