As a followup partially to chromatic's note: the mod_perl guide suggests using Apache::GTopLimit instead of (or in conjunction with) MaxRequestsPerChild. The reason is that it provides even more fine-grained control over when your webserver child exits, and it allows you to set memory/process limits, rather than playing guessing games depending on how many requests a child has received.

Check out the Improving Performance by Prevention section of the guide. Particularly:

• Keeping the Shared Memory Limit, which describes how to kill a server when its memory becomes unshared to a certain point; and
• Limiting the Size of the Process, which shows you how to set a maximum memory size for your child process.