If you're using FreeBSD, you don't need to rely on the CPAN module for installing common modules. So, first make sure you don't already have libwww installed. pkg_info -I -x libwww. If you do, deinstall it. Then install it fresh, portinstall p5-libwww. After that, you'll be able to keep it updated with portversion. portversion -vR p5-libwww will check the version of not only libwww, but all modules upon which it depends. If you can find some that need upgrading, you can tell portversion to make it's output a usable shell script, which you can pipe right to sh.

portversion -vR -x perl -c libwww | sh

I recommend the -x perl so it doesn't just go ahead and upgrade perl itself whenever the perl port gets updated.