Is installing perl5.10 on CentOS 5 by rebuilding the Fedora source rpm's a good idea?
Also, isn't is bad that CentOS still does not support perl 5.10 after so many years?
It's NOT a great idea. The reason is that if you have a Perl module with binary code (such as DBD::mysql; the database driver for MySQL, but there are much, much more); they will not be binary compatible between 5.10 and 5.8. This means that if you use Perl 5.10 you'll also need to get DBD::mysql from Fedora, which might require getting the Fedora Mysql libs....
The most 'Modern' way to install a recent version of Perl is by using perlbrew; this sets up a Perl in a users' home directory and separates it in such a way that it does not mess up the system perl. The drawback is obvious; it is on a per-user basis only.
On the question if it's bad if CentOS 5 still does not have a perl >5.8: what do you expect from such an old operating system? If you want a newer perl in your operating system, please wait until the nice folks at CentOS ever release CentOS 6, or shell out your good money to get RHEL6, or get Scientific Linux 6, or get Fedora (which is not to be used on production servers, as it's state-of-the-art/bleeding edge and has a very short support span). Of course you can also consider non-rpm-based distros such as Debian or Ubuntu LTS; which all have recent perls.
If it compiles, and you need it, it may be.
CentOS strives for binary compatibility with Red Hat Enterprise Linux (RHEL).
Red Hat strives to please corporate customers. Companies don't appreciate breaking their application by an operating system changing version of an important program.
perl 5.10 is in RHEL 6, so it will be in CentOS 6, whenever that one appears...