Technically speaking, PERL is an acronym for Practical Extraction Report Language. As such, perl has great capabilities for text processing, pettern serching, and other text related utilities. None of these are OS dependent.

There are obviously lots of modules available for perl as well. Most well-written modules will be OS independent as well. A good example that come to mind is Tk, which is a package to create GUIs.

At the same time, there are a number of Windows specific modules. Those are of course specific to only that OS (examples would be Win32::GUI and Win32::API). Sometimes, if you want to program on windows you have to rely on those modules, which makes the code platform specific.

There are a number of other, in my opinion, small variations between the platforms (i.e. implementation of fork(), functionality of Time::HiRes are just a few examples which come to mind).

However, it has been my experience that most limitation are actually imposed by the user (i.e. usage of system() calls) and NOT the available modules. With that being said, if your intent is to learn Perl, you could do that on any OS.