Perl is the most platform-agnostic language I've ever come across or heard about, with the remotely possible exception of a few FP languages like Haskell. It just works everywhere, from windows to *nix to mainframe to palm.

Now, learning an OS isn't that hard, either. Every OS has files, directories, a kernel, memory management, and other basic items. The only difference is in how you, the programmer, relates to them. Some, like the *nixen, tend to trust the developer a little more. Others, like the mainframe, tend to trust the developer a little less.

Me, I'd shoot for being comfortable on both *nix (which includes FreeBSD, Linux, and Solaris) and Windows. Mainframe is if you need it. I've programmed on nearly every *nix variant out there and, frankly, they're practically identical with respect to installing modules and programming stuff. Learn a shell, an editor, basic vi, and how to drive gcc. Beyond that, learn what you need to get your job done. Threading can be a bitch in various *nixen, so I tend to avoid threading. (Frankly, it's a bitch in most situations and you very very rarely will need the potential performance boost.)

Windows is more difficult. Learn how to use PPM and Notepad. Beyond that, get used to frustration because AS perl is not exactly the same as regular Perl. It's close, but has a few differences, particularly in performance of fork and how terminals are emulated when done through select.

perldoc perlport is your friend, here.