You can install Perl 5.8.5 directly from CPAN as well.
There is only one change you need to make in the default settings to make it work with cpanel.
Code:
cpan
install N/NW/NWCLARK/perl-5.8.5.tar.gz
Accept all the default answers EXCEPT the following:
Installation prefix to use? (~name ok) [/usr/local]
Instead of pressing ENTER/RETURN to accept the default you MUST type:
/usr
From there on, you can accept the defaults.
If you're going to upgrade, you might as well get the latest version.
I'd also run:
after the Perl upgrade to make sure CPANEL has all the appropriate Perl modules it needs for the new version.