perl: make File::Spec treat msys as Window

Since our Perl installation is there to cater to Git for Windows,
which is not an msys program, it would be better for it to treat
paths as Win32 paths. So let's tweak it to do just that.
Revert "perl: make File::Spec treat msys as Window"

This reverts commit eeaeecbbefd412e4d19b77f662c1a9d2d32bf43a.