AnyEvent for concurrency; no threads aside from the fork that causes child workers to pick up a message. The validation is in the parent, the child uses the data. The race would have to be something mutating the array, but I can only find one place where mutation happens. That part is a tight for each loop that appends source data to the target array. I will likely have to add logging, but the issue is so infrequent that the extra logging will be a mess due to the volume of data this service handles (around a billion requests per day, and maybe between zero and 200 requests will face this issue). Something will work out eventually; the bug must be exterminated ;)