Thanks haukex, good to know what the mechanism is, though I sort of figured it would be presumptuous of me to suggest it officially without asking the Monastery first, which is why I did.

Of course, another way to prevent your bug would be to be more careful with the lexical scoping of filehandle variables

Yes, that is most certainly the best solution. I have this habit of declaring all my filehandles in the global scope and opening them all right away as a first step in my programs just to force the user to make sure all the the input files are accounted for before my program goes off and spends a bunch of time processing some of them only to find later one or more is missing and it has to die anyway. It's probably an amateurish thing to do.