Did you Google this? There are many discussions about it on the mod_perl list. Here's the summary:

• You normally don't need this, unless your database really can't handle the number of connections it would take to keep them persistent from each process. This is almost never true with MySQL.
• If you're running mod_perl without a reverse proxy in front of it, do that first. It will probably solve the issue.
• If you still need to reduce the number of connections after that, look at DBI::Gofer or sql-relay.