I wouldn't presume to know which would be the best platform for you to learn Perl on...
... but I am fairly certain it isn't XP (or any other MS toy OS.)
If you are really considering the long-term employment implications, consider that the employers/clients you find using Solaris are likely to be culturally different than those using Linux or FreeBSD. And the same can be said about the Linux shops versus the FreeBSD shops, for that matter. I've found a personal preference for cultures that embrace Linux. I still have a lot of respect for FreeBSD from a technical perspective though.
I have worked in all three cultures, by the way. If I had to describe each in one word I'd say... Linux is tribal, FreeBSD is agrarian and Solaris is feudalistic.
-sauoq
"My two cents aren't worth a dime.";