Ran into a similar problem. In researching this I found a posting on the Apple Developer site that indicated the latest Xcode (3.2) was needed for cpan to compile stuff and put it in the right place for Perl 5.10.0.

You may also be running into problems with 32-bit vs 64-bit kernal. Reboot your system and hole down the 3 and 2 keys to put kernal in 32 bit mode.