open SCRIPT, "some_script |";
open OTHERSCRIPT, "some_other_script |";
open INPUT, ">&SCRIPT";
while (<INPUT>) {
  if ($some_condition) {
    close (INPUT);
    open INPUT,">&OTHERSCRIPT";
  }
  do_something_with_input();
  if ($some_other_condition) {
    close (INPUT);
    open INPUT,">&SCRIPT";
  }
}