Code:

while read d1
do while read d2
   do while read d3
      do while read d4
         do while read d5
            do # whatever
            done < f5
         done < f4
      done < f3
   done < f2
done < f1

I told you exactly what to do in post #2 in this thread. Above is an example of how to waste time with five file descriptors. Feel free to increase this to a thousand file descriptors if you want. I make no guarantee that any shell will work if you nest this to 1000 levels instead of 5. And, I see no reason why doing so would be advantageous in any real-world application.