my $pid1 = open(PIPE1, "myprog1|");
  my $pid2 = open(PIPE2, "myprog2|");
  while ($pid1 >= 0 || $pid2 >= 0) {
    my $pid = wait;
    if ($pid == $pid1) {
      print "myprog1 completed.\n";
      $pid1 = -1;
    } elsif ($pid == $pid2) {
      print "myprog2 completed.\n";
      $pid2 = -1;
    }
  }
  my $output1 = <PIPE1>;  # this works (I see the output of "myprog1")
  my $output2 = <PIPE2>;  # this works as well
  close PIPE1;  # this fails!
  close PIPE2;  # this fails as well!