The problem is probably the order of paths in PERLLIB in the Zimbra users environment. Change the order so perl looks in your systems perl install for the needed modules first, and it will all work.
I've got Zimbra running on RHEL4 64bit and everything works as long as you:
1.) make sure all the needed perl modules are available in 64bit perl (install them all from CPAN)
2.) change the zimbra user's environment to use those modules, instead of the ones included with Zimbra.
Do a 'find . -name *.pm' in /opt/zimbra and you can get a good idea of what you need in your system perl paths. |