Because Learning Perl takes a Unix slant to things. Win32 is sufficiently different that it makes sense to have both. Specifically, there are a huge number of Win32-specific modules that have little to no analogue in the Unix world.
That makes me wonder why there isn't a Learning Perl for Unix, or something like that.
Actually, I think it's a good idea to favor non-platform-specific texts, but that makes me want to again question the Win32 version's existence. Shouldn't "we" be teaching Perl, and maybe offering a "next book" that touches on Win32-specific matters and assumes a basic understanding of Perl already, instead of teaching a Win32-specific dialect of Perl?
I'm not sure what you're asking for: Learning Perl isn't tied to any platform, and Learning Perl for Win32 is a very old book that won't be updated. We're (as in Stonehenge and the authors of Learning Perl), don't teach a platform specific version of Perl. Learning Perl for Win32 was a reaction to the second edition of Learning Perl where Tom Christiansen let his Windows hatred (and Unix bias) get in the way. Tom Phoenix changed that in the third edition of Learning Perl so we didn't need a Windows version of the book.
A Learning Perl for Unix wouldn't be all that interesting. The interesting parts are where Perl, which is based on a lot of unixisms already, differ on non-unix platforms. Most of the interfaces have been faked or kludged so they work most places already.