If you are going to try it you do it at the perl5 level. So start by doing mv /usr/lib/perl5 /usr/lib/perl5.old so you can roll it back if required. Also the actual perl binary can end up in a number of locations depending on what you do at compile time. Do a ll `locate bin/perl` or similar to find which are symlinks and whare your real binary is located. You need to rename and replace that as well.

I don't particularly recommend this. I would just get the latest 5.8.x and recompile it. You can leave the existing install inplace.....