In my experience, regardless of unix-like platform, you don't want to upgrade CPAN. At least, not in-place. With the system perl. Often that drags in a bunch of other changes, such as a newer perl, and upgrading the system perl is often a bad idea. For starters, you'd need to be root, which you may not have. Second, it'd break the fixes that come from the vendor (in your case, IBM). Third, it may break other apps that come with your system (anything written in perl that comes with AIX might break with an upgrade of perl). Any one of these reasons, by themselves, is sufficient reason to avoid upgrading the system perl, IMO. Especially as AIX 6.1 comes with perl 5.8.8, which is not going to be ABI-compatible with anything newer (other than 5.8.9, but that's not what you're going to end up with).

Instead, I would recommend installing a new copy of perl to another location. I've just gone through putting together a perl script that automatically installs perl 5.16.0 plus 56 other modules (many involving XS code) to a new location, and I've been testing it on AIX 6.1 with xlC, so it seems to work. Some of them are even a bit more esoteric (Coro, AnyEvent, EV - though I had to patch this one, it won't compile with xlC without a patch that the author has accepted but has refused to release). That will also get you pretty much the latest-and-greatest CPAN at the same time.

I've also had issues with Term::Readline::Gnu on various platforms (including Linux!) from time to time. So I generally just use Term::Readline::Perl - it's big, ugly, and more or less just works. It's not quite as feature-rich as ::Gnu, but I hadn't noticed until someone pointed out to me some of the programmatic differences.

Whatever you do, don't ask Tux. He'll try to convince you that HP-UX is an upgrade. Ewww! :-) (Sorry, long-running fun-poking going on.)

Edit: a couple minor tweaks, but I also want to add one other thing. Getting all those extra modules to compile? They work better if you add -Accflags=-qlanglvl=extc99 to the Configure command. It's not needed for perl itself, but is very handy for many XS modules whose authors write to the C99 standard instead of C89.