in reply to
Re^4: cpan fails when MKS installed on Windows (HKCU\Software\Microsoft\Command Processor)
in thread [Solved]: cpan fails when MKS installed on Windows
everything is affected, its a registry hack
Clearly $^X is not affected.
I think the OP's problem might have no relation to the presence of MKS. Instead, it could be the Makefile that ExtUtils:MakeMaker has generated - which perhaps (or not) goes back to a %Config setting.
Over the years, I've seen similar errors where MKS is *not* present, but I don't think I've ever quite got on top of how the error arises, or precisely what to do to fix it.
I'm referring to a problem that turns up from time to time with DIRFILESEP.
If the OP is still interested, he might verify that the Makefile generated during the building of the module contains:
DIRFILESEP = \\