I had this problem with ActiveState Perl when I actually did install Perl in D:\Perl, and then later in C:\Perl. Their installer apparently doesn't clean up the registry when you uninstall. The upshot, if I remember correctly, was that it sometimes thought it was installed on C:, and sometimes on D:. Or, rather, the executable knew where it was, but the configuration was screwed up and sometimes modules or the PDK got installed in the wrong place if I wasn't paying attention.

If you want to fix the problem permanently, first uninstall Perl and any add-on programs such as the PDK, then physically remove all Perl directories you can find. Then use regedit to track down and remove all references to ActiveState in the registry. After that, things should be normal.

To see the path, type "path" in a command shell. To alter it, right-click on My Computer, then Environment Variables. Locate PATH in the lower window and edit it. Be careful with your semi-colons or you can mess up your configuration.