I'm having trouble with the foreach within a foreach loop. I can't figure out how to do this effectively.

The trick is not to use two nested loops. Instead use the session ID as a key in a hash, and then when interating through @array, extract the session ID from each line, and check if the session ID exists in a hash. That should reduce the runtime from O(@array * @sessionID) to something more along the lines of O(@array + @sessionID).