You need to recompile XS modules in either case. The CPAN module has some utilites to take care of that. Even so I had to compile/install some things outside of the CPAN shell before I got things to the point that CPAN's tools would work.

If it's a hobby box, I'd say go for 5.8.0 it's been mighty stable for me, and it has some nifty new features.