I doubt that you have to worry too much about it, so long as you avoid any ms-windows-specific modules and practices. You might need to worry about portability of data files more than portability of code.

Bear in mind that macosx is really BSD unix (and so is really not noticeably different from linux), as far as perl is concerned. (MS-Windows is the odd one.) So if you have access to any sort of unix/linux box, and can test your script there, that will do for testing the macosx environment.

When in doubt, post some code here, so monks with macs can try it out and let you know what happens. (And, as suggested above, consult perlport.)