I had a problem similar to this in my work application a while back, which I eventually tracked down to a bug in perl that was being triggered by the complexity of the operations inside the sort comparison subroutine.

I don't remember the precise nature of the bug, but it was triggered in my case when the method invoked in the sort needed data not yet fetched from the database.

I worked around it by adding an extra pass through the list to ensure the method was called once on each object (and the data from the database was therefore already cached) before the sort itself, for each of the affected sorts. My checkin message at the time said "add temporary fix to avoid sort subroutine scoping problems that affect all perls earlier than 5.8.0".

Hugo