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

As the utf8 manpage puts it,:

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

And in the perlunicode perldoc:

           WARNING:  As of the 5.6.1 release, the implementation of Un
+icode
           support in Perl is incomplete, and continues to be highly e
+xperimental.
[download]

So, I guess there are problems using extended unicode and that's why it's marked experimental.

Aziz,,,