You have installed a PPM for Perl 5.12. This cannot work with Perl 5.6.
If you want to help us help you better, you could tell us the error messages you get when compiling Win32::API. That way we can see where it goes wrong for you. Looking at the Changes for 0.75, it seems that at least version 0.75 (again) supports Perl 5.6.
As an aside, in your case, I would really look at upgrading Perl to a more recent version, especially for new development. ActiveState Perl and Strawberry Perl offer recent versions that already include Win32::API.