I have pretty the same situation: debian sarge, and no will to use perl 5.8.4. I ended up in installing perl 5.8.8 into /opt/perl, and using only CPAN to install new modules in the new perl system (as a matter of fact, I keep the old perl only for scripts that may actually need it, in particular those in the debian system).

Even if there should be binary compatibility between the two versions of perl, I'd refrain from using modules from the 5.8.4. What if you need more updated modules? I've simply installed the modules again in the new tree and let them live their lives.

In a previous installation (using Slackware) I recompiled perl as well, jumping from 5.8.0 to 5.8.4. In that occasion, I played a dirt trick and made /usr/local/lib/perl5/site_lib/5.8.4 point to /usr/lib/perl5/site_lib/5.8.0 without having problems, due to binary compatibility. But I think that was a quite different situation: I have never truly used Slackware package system, so I knew I wasn't going to spoil anything installing new modules in the old perl tree.

Hope this helps,