use utf8; $_="foo"; /\X/   # runs fine

##</code><code>##

       WARNING: The implementation of Unicode support in Perl is
       incomplete.  See the perlunicode manpage for the exact
       details.

##</code><code>##

           WARNING:  As of the 5.6.1 release, the implementation of Unicode
           support in Perl is incomplete, and continues to be highly experimental.