I must say I've not used much this module in actual "production code", especially since it requires a patch to the sources, but it is real fun to play with. Also, it's totally non-invasive. I wish it could make its way into a next major release...
This module is really a little revolution to Perl5 and has been heatedly discussed as such among the perl5-porters. It lets you treat everything as an object and thus makes Perl5 look very much like Ruby (or rather any language in which primitive types are objects).
The only drawback is that you need perl-5.8.1RC4 and to apply the enclosed patch against the source-tree. So essentially you have to compile your own Perl.
However, I haven't had so much fun with Perl for a long time. Once it makes it into the core (Perl5.10 is a candidate) a whole new branch of modules will probably make it to the CPAN.