i find it interesting that using PPM supplied with ActiveState's Perl, none of these PM's showed up.
Inline.pm looks like a good solution if we were rolling out our packages (internal term, not Perl packages) to machines that we knew had C compilers on them. in the Fortune 5000 world, i think this does not have a high probability. :) very interesting, though and i'll have to see if i can put into some development stuff.
FFI looks to almost be more clunky than Win32::API. look at this chunk of code from the module's examples section:

$clib_file = ($^O eq "MSWin32") ? "MSVCRT40.DLL" : "-lc";
$clib = DynaLoader::dl_findfile($clib_file);
$strlen = DynaLoader::dl_find_symbol($clib, "strlen");
$n = FFI::call($strlen, "cIp", $my_string);
DynaLoader::dl_free_file($clib);
[download]

looks like a lot of work IMO.
C::Dynalib looks good, but it appears that it still requires some knowledge of where the functions you want to call are located.
it looks like for our use, Win32::API might just be the best option. we will be using several registry calls which are fairly well supported already, and after some design talks it looks like we'll only need to call one method through Win32::API, which shouldn't be too bad. if it were larger scale, Inline.pm looks very good.
thanks for the excellent info!