Those three lines do not help debugging. Better to let us see how you form the $mess_line.
By the way, you are appending to the end of the old file by sepcifying ">>", but I think that might be just what you want to do. Depends on what you have in the $mess_line, is it "new content only?", or "old+new"?
Well, difficult to blind debug, give us even those what you thought trival.