What compiler are you using? I got it to work in 64bit on AIX5.1 using this Configure stream:
sh ./Configure -Dcc=xlc_r -Dusenm -Dprefix=/appl/datasync/work/perl5 -Dusethreads -Duse64bitint -des
-Duse64bitall will also work.
I used vacpp 6.0, and also got it to work with gcc3.3.3