I would like to build this meditation on the old saying that two heads are better than one, and say that two OS's are better that one.

That's the spirit. One of the great attractions of Perl is that it is available on a multitude of platforms. This means that it is possible (and usually desirable) to write portable code.

However, the argument that because perl itself is portable, programs written in it are portable, is a fallacy. Consider hardcoded references to "/dev/tty" or "C:\Program Files".

It's also not just a compatibility issue between Unix and Windows. I learned Perl on VMS, which has a vastly different syntax for filenames. Also note that the Apple Mac (prior to OS X) uses colons to delimit file paths.

Filename syntax is only one portability issue. For the full monte, see perldoc perlport, supplied with the distribution. I have also given a talk on portability recently, with slides.