Perl 6 for Perl 5: Revision 2
Incredibly, an effort lead by Flávio Soibelmann Glock is working to actually implement Perl 6 in Perl 5. You can download v6-alpha now on CPAN to try it out. It's feature set has not caught up with pugs yet, but there is steady progress on it.
Perl 6 features are also coming to Perl 5 in other ways. As described below, a number of Perl 5 modules already implement some Perl 6 features, and some aspects of Perl 6 are being added to the official 5 releases, starting with Perl 5.10.
Perl 5 modules implementing Perl 6 features