Ubuntu is an excellent distribution, though it is targeted more toward "easy desktop environment for people who don't know Linux". This is either excellent or horrible, depending on your personal taste and level of experience.

It's based on Debian, and has done so many good things that the Debian team is going out of its way to establish and maintain good relationships with the Ubuntu folks, so that they (Debian) can borrow some of the nifities that Ubuntu has come out with. (Personally, I use Gentoo, but that's not a distro I'd recommend to someone who isn't a bit of a masochist, like me).

I, too, ++ to Ovid's comment. However, I would add one note of caveat emptor: Perl itself and many of the nifitiest CPAN modules were originally conceived of, developed, and tested for UNIX-like OS -- ActiveState or PxPerl on Windows are just fine, but a UNIX-like will have fewer limitations and oddities to deal with. I'd recommend using a UNIX-like OS (Linux, BSD, etc.) if you have or desire *NIX experience. Use Windows to learn Perl on only if Windows is the only OS you're really comfortable with.