Though it's certainly well intentioned and not unreasonable to draw attention to the content of the "user list" data file, I can't imagine how the OP code could be getting an "undef" element in the @nup array.

Have you ever encountered a situation where @array = <FILE> produces an undef element in the array? What sort of condition in a data file would cause that?

I think ig's question below is more likely to be on the mark.