Sounds like you did a manual install rather than use PPM or similar?

The three files, Type.pm, Struct.pm and Callback.pm should be located in a subdirectory, API within the Win32 directory. This package is normally added below perl/site/lib rather than perl/lib though thats more convention than required.

To clarify, this is what the directory structure should look like for the whole Win32::Api package starting at lib directory (whether this is perl/lib or perl/site/lib).

lib/Win32/API.pm
lib/Win32/API/Type.pm
lib/Win32/API/Struct.pm
lib/Win32/API/Callback.pm
lib/auto/Win32/API/API.dll
lib/auto/Win32/API/API.exp
lib/auto/Win32/API/API.lib
lib/auto/Win32/API/Callback/Callback.dll
lib/auto/Win32/API/Callback/Callback.exp
lib/auto/Win32/API/Callback/Callback.lib
[download]

Unless you have particular reasons for not being able to use PPM or CPAN to install packages, then you will probably find one of these methods much simpler. See tachyon's A Guide to Installing Modules for an introduction if you havent tried this before. Start by looking at the section entitled "PPM".

There are other, possible more detailed examples of using PPM around the Monastery, but this was the first one Super Search turned up.