This is a multiprocess application with an SQLite database. Have you considered moving up to PostgreSQL? The agent processes could insert their records into the import table and issue NOTIFY to report completion to the main worker process, which has issued an appropriate LISTEN and uses the pg_notifies method to receive the notifications. (See DBD::Pg for details.)

As an extra benefit, PostgreSQL LISTEN/NOTIFY is integrated with the transaction system: a notification sent during a transaction is held in the database server until the transaction commits.

The catch is that you will need to be careful with your DBI handles: each process needs its own database connection and the "agent" processes must not use the "worker"'s DBI handle by mistake.