Since I have been asked, let me briefly elaborate ...

Every version of OS/X has some “standard” version of Perl installed, and in the case of the Snow Leopard --> Lion upgrade it was actually quite a switch.   As is the case with every Unix-like operating system, you are sure to encounter some software (vendor-supplied or otherwise) which has been tested against that particular version of Perl, such that there might be very serious consequences indeed if it were changed in any way.   Therefore, you can’t change it; you mustn’t.

But you can do what you can do in the garden-variety situation that has been re-hashed many times in the scenario of needing to run a different version of Perl than your shared-hosting provider uses.   You install your chosen version of Perl locally (e.g. in /usr/local or in your home-account), perhaps by compiling a source distribution directly.     One of the things that you can do at this point is to set the “built-in” set of @INC directories, and here you probably want to configure your local Perl executable so that it does not include built-in references to the system-wide directories at all.   You compile and install this from-source version.   Then, you arrange for PATH to point to this Perl binary in preference to your own.   Finally, you arrange for PERL5LIB to point to a local directory as “your CPAN repository,” and you set up a CPAN configuration to refer to that directory.   Now, you install into this location everything that you need.

What you have now done, as long as both PATH and PERL5LIB are properly set, is to create an entirely-isolated Perl installation that not only consists of precisely the Perl version that you wish, but also that refers only to the CPAN libraries that you wish for it to see, as though nothing else existed.   (Because, from its point-of-view, they don’t.)

On OS/X or any other system, I always define one (non-privileged) user account as “the user-level software maintenance account,,” such that I do all such software maintenance using that account (and do nothing at all there, other than that).   “Local software” directories are read/write accessible only to that user.