As roboticus says - open two or more database connections (or client apps) and run requests with varying degrees of delays.

I presume that the transactional part of the system is handled at the database level. Even so, if Dbix::Class offers a high level of abstraction over what actually happens in the DB (something I don't know as I haven't used it) then you should be careful of things like deadlock situations where locks on different resources are acquired in a different order in two processes, and each process is waiting to acquire a lock that the other already holds.

And keep in mind that you are likely to find race conditions in places where you didn't think that they were likely to appear...

Michael