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

         ...

         _utf8_on(STRING)
           [INTERNAL] Turns on the UTF-8 flag in STRING.  The
           data in STRING is not checked for being well-formed
           UTF-8.  Do not use unless you know that the STRING is
           well-formed UTF-8.  Returns the previous state of the
           UTF-8 flag (so please don't treat the return value as
           indicating success or failure), or "undef" if STRING
           is not a string.