Others have covered most of the information that I could share on the topic of your post.
About half the PCs are XP/x86, and the other half are Windows 7/x64. Some CPAN modules work on x86 but not x64 (e.g., Win32::SerialPort).
It's been my experience so far that the Win32 modules that I've used on 64-bit Windows work just fine. I guess that could change with future Windows operating systems.
Also, since I personally don't have any scripts needing features like 64-bit integers from 64-bit Perl, I've been using 32-bit Perl on 64-bit Windows without any issues so far.