1. Use the modulus operator (perlop).
2. What about using one loop and putting the statements after each other. Pseudocode:

   open File1
   open File2
   while (condition) {
     if (read from File1) {
       do something
     }
     if (read from File2) {
       do something;
     }
   }
   [download]

3. Have you tried it? What does it do?

-- Hofmator