I actually said:

Since there is no mention of "MessageBox" in any of the packages Perl sources,

I grepped the source files (*.pm/*.pl). As the document you mention is .pod, it was not found.

When I try to use the htmlised docs for Win32::GUI, all the links are dead ends producing:

You tried to access the address file://localhost/c:/perl/html/TEMP/Win
+32-GUI-1-1300-1139047283/blib/html/site/lib/Win32/GUI/Reference/Metho
+ds.html, which is currently unavailable.
[download]

which means that I did not see the info you've cited.

There's probably some magical invokation of some obscure AS module that isn't documented, that would cause those .pod files to be htmlised and linked properly, but I am not aware of it.

And when I try getting to it from the cpan docs I get

Not found

Whatever it was you were looking for, it's not here. This may be due t
+o... 
You made a typo in the URL
What you are looking for no longer exists on CPAN
[download]

If CPAN doesn't know how to treat .pod files, I'm not surprised that I don't.

Having an optional first parameter on a method seems a little obscure?

A quick check with PPM finds Win32::API::Prototype:

ppm> search Win32::API::Prototype
Searching in Active Repositories
  1. Roth [http://www.roth.net/perl/packages/] Win32-API-Prototype [0.
+2002.12.17]
                Roth Consulting (http://www.roth.net/)
ppm>
[download]

Google turns up many similar pointers.

There is a Win32::API mechanism similar to Win32::API::Protoype::ApiLink(), namely Win32::API->Import(), hence I linked to that module, but I've found the former less buggy.

Glad you found your solution.