You will want to use the Apache::DBI module only if you are opening just a few database connections per process. If there are ten child processes and each opens two different connections (using different connect( ) arguments), in total there will be 20 opened and persistent connections.
This module must not be used if (for example) you have many users, and a unique connection (with unique connect( ) arguments) is required for each user. You cannot ensure that requests from one user will be served by any particular process, and connections are not shared between the child processes, so many child processes will open a separate, persistent connection for each user. In the worst case, if you have 100 users and 50 processes, you could end up with 5,000 persistent connections, which might be largely unused. Since database servers have limitations on the maximum number of opened connections, at some point new connections will not be permitted, and eventually your service will become unavailable.
That is, database user connections. This doesn't mean that if many people register as users on your web site you shouldn't use Apache::DBI; it is only a very special case.
If you want to use Apache::DBI but you have both situations on one machine, at the time of writing the only solution is to run two mod_perl-enabled servers, one that uses Apache::DBI and one that does not.
In mod_perl 2.0, a threaded server can be used, and this situation is much improved. Assuming that you have a single process with many threads and each unique open connection is needed by only a single thread, it's possible to have a pool of database connections that are reused by different threads.