This almost certainly has to do with how you installed the module,
so how did you install it?
It usually happens when you're missing the DLL library linked to a Perl
module, and that happens when you installed the module copying just
the .pm file.
To install "properly", in the case of Win32::API, you must have a
compiler installed, or use the Activestate PPM file for Win32::API if
you're using Activestate, or use Strawberry Perl, which I recommend.
Also try this search:
http://duckduckgo.com/?q=Can%27t+locate+loadable+module+perl
And this one: