There's some partial Win64 support in the github version:

git clone git://github.com/cosimo/perl5-win32-api.git win32-api
[download]

I actually have an API_test64.dll (different to the one on github) that allows t/API.t to pass its tests - but I think the partial support is for MS compilers only. Nothing there for x64 Strawberry Perl, afaik.

Here's the best I can do with that github version:

t/00_API.t .......... ok
t/01_Struct.t ....... 1/7 # $^E=
t/01_Struct.t ....... Dubious, test returned 5 (wstat 280, 0x500)
Failed 2/7 subtests
t/02_GetProcess.t ... 1/3 # $result=3980
# $$=3980
t/02_GetProcess.t ... ok
t/03_undef.t ........ # Win32::API 0.60_64
t/03_undef.t ........ 1/3 # Import: The handle is invalid
# PeekNamedPipe: The handle is invalid

#   Failed test '(RT \#39730) sample API (PeekNamedPipe) works with un
+def values
'
#   at t/03_undef.t line 60.
# Looks like you failed 1 test of 3.
t/03_undef.t ........ Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/3 subtests
t/04_rt_48006.t ..... 1/3 # Size is 296. Should be 296
t/04_rt_48006.t ..... ok
t/05_more_struct.t .. ok
[download]

It may be possible to do better with some simple hacking, but I don't pay much attention to Win32::API any more.
Imo, if you want to access the Win64 API, best to do it with XS (or Inline::C) - and that goes for both ActivePerl and Strawberry Perl.

Cheers,
Rob