I have just checked in some changes that will allow the building of Win32::GUI with 64 bit perl. The resulting dll's are all 64 bit meaning they run native on 64 bit machines with all the advantages (access to more memory and increased performance). At this stage, the changes should be considered the first draft but so far I haven't found any issues in testing. Some of the modules don't compile with mingw as they rely on MS frameworks such as Win32::GUI::Grid.
I would be grateful for any reports of issues/problems. These changes have been tested with the 64bit version of mingw not with the 64 bit version of the Microsoft compiler, but that should also work. The changes should also be compatible with any 32 bit compiler.
Strawberry perl has started to ship a version with a version of 64 bit mingw built in:
Once downloaded and installed (on a 64bit machine!) it's just a case of getting the source from sourceforge and running Makefile.PL. If you have a problem getting the source, drop me a mail off list and I'll email it to you.