I haven't followed all of your travails up to this post, but you may want to try the Windows environment variables. In Windows 2000, if you right click 'My Computer' and do Properties, click Advanced, then Environment Variables, you can set the base working path for your script as a system variable (or a user variable, whatever you deem appropriate).
This node seems to have a good discussion of using Windows environment variables. Of course, it seems like it would be easier to me if you put the base bath in a Perl variable, but its up to you. :)