file a      file b

1 read a line from a
2 read a line from b 
3 read a line from a
4 read a line from b
...


##</code><code>##

while(<F1>){
  #read a line
  while(<F2>){
     #read a line
     last; # go back to the main loop, but how to continue from this point in the next iteration ?
  }
}