I think I'll take a look at Win32::API anyway, because there are probably lots of things you can do with it that aren't covered by any Win32 modules. How do you find out what API calls you can use and what format they require? I suppose we have to trudge round the MS site to find clues?
Please check out FFI and C::Dynalib as well. They are not restricted to working on just Win32 and seem to have cleaner designs (though the APIs could use the addition of some helper functions to make things simpler).
Documentation of many APIs in Microsoft DLLs can be found by downloading the SDK (Software Developers' Kit) from Microsoft or buying a compiler. You might be surprised how many APIs are already available from Perl via modules.