I have been relying on the correctness of the module to check the various things mentioned.

Um, File::Copy::move()'s documentation says nothing about "Waits for the source file to be finished being created and then...".

Trying to get the monitor/copier process to correctly determine whether the to-be-moved file is finished being written is likely a hard problem, perhaps an impossible problem (depending).

That is why I outlined the "usual" way to have whatever is dumping these files into the source directory to do it in a way that makes it clear when the file being dumped there has been completely dumped (and is thus ready to be moved by the monitor/copier).