Module::Build cant upgrade anything XS it uses (or couldnt/cant without a modern EU::I) on Win32. So since Pathtools is a dual build module, the only way you can install it on Win32 is to install it from a clean shell with nothing running, and then to use the EU::MM install infrastructure as it avoids locking the files (by not using Perl to do the install).

Holy shit.まじかい。