If you're using Jaguar (10.2) here are complete directions from Apple's site for upgrading to Perl 5.8:
[developer.apple.com...]
1. You don't need to un-install 5.6
2. 5.8.3 works great on Panther
3. Follow timster's tip
The only thing I would say about the instructions he refers to is this. Although it says to download the distribution from the command line you don't have to. I just downloaded it from CPAN, unstuffed it on the desktop and placed the resultant folder into /Library/Perl moved into that directory using Terminal and did the install as instructed. You will need to install developer tools to be able to make the install.
Good luck
Sid