A standard Perl script using DBI will automatically perform a rollback whenever the script exits. In the case of persistent database connections, the database handle will not be destroyed and hence no automatic rollback will occur. At first glance it even seems to be possible to handle a transaction over multiple requests, but the temptation should be avoided because different requests are handled by different mod_perl processes, and a mod_perl process does not know the state of a specific transaction that has been started by another mod_perl process.
In general, it is good practice to perform an explicit commit or rollback at the end of every script. To avoid inconsistencies in the database in case AutoCommit is Off and the script terminates prematurely without an explicit rollback, the Apache::DBI module uses a PerlCleanupHandler to issue a rollback at the end of every request.