The very first thing that I do with a legacy application that uses mod_perl is to switch it to Plack. The conversion process is actually very easy.
Plack uses the FastCGI mechanism to run the Perl code in a stable of separate worker-bee processes which can be managed by (Apache) itself. The critical difference is that the Apache worker-processes are once again “lightweight,” and cannot themselves be ruined by a (Perl) application that doesn’t work, because the work is actually being done by a worker process that is held at arm’s length. It is also very easy to offline-test the applications through the provided plackup command.