I would also uninstall the rpm version, but you certainly can keep the older Perl and install a new version next to it. You will need to know where the Perl library tree is located.

perl -e "print @INC;"

will list off uncerimoniously where the library is. Perl will normally install it's libraries in /usr/local/lib/perl5, unless you use -Dprefix= during configuration. The rpm version of Perl on my ISP uses -Dprefix=/usr and lands in /usr/lib/perl5.

For installing Perl 5.8.0, after the necessary download, gunzip,and "tar xvf" commands :

1. sh Configure -de -Dusethreads (-Dprefix=/usr maybe)
2. make
3. make test
4. make install (as root)
5. installman

After it's all done, you may see some 5.6.1 or other older version directories embeded next to 5.8.0 in the Perl tree. These can be removed at some later date, but be careful to keep a tar/gzip copy around in case somebody is linking into this part of the tree (like maybe a static apache module).

Good Luck!

Update

The location of the executable, "which perl" shows where it is, may need some adjustments in the name. The installation creates a file called perl5.8.0, but it would be better to name it simply perl. The previous Perl executable will have this name and take precedence until it get replaced. Use "perl -v" to check the version.