Sorry, I thought you were the OP. Setting the private attribute prevents connect_cached from replacing the existing dbh in its cache with a new one. It is intended to help with forking, where you don't want to trigger a DESTROY call on the old handle because it may tell the server it's disconnecting and thus screw up the connection in the parent process where you want it to work.

Working with a forking server is really tricky if you want the handle in the parent to keep working. I suspect you missed a connection when you set InactiveDestroy. If, for example, you use a module like Proc::Daemon, it does a fork of its own, and you muse set InactiveDestroy on the handles it opens before forking, or they will break things later.