@data starts out with 1 element: "" (the empty string), from when you initialize it. $data[1], $data[2], $data[3], and $data[4] are all uninitialized.

As you go through the last foreach loop, you (might) assign $_ to specified elements of @data. On line 49, you print all the elements of @data - some of which are obviously uninitialized.

You can initalize them with something like the following:

  my @data = ('','','','','');
[download]

To have it start it's life with 5 empty strings.

Also, consider moving the open/close statemens outside of the foreach loop.

  open STDOUT, ">>", "$output0" or die "$output0: $!\n";
  foreach (@list) {
    ...
    print STDOUT, @data;
  }
  close STDOUT;
[download]

Since you're appending to the output file, you might as well just open it once and be done with it.

Update: Fastolfe pointed out that the initialization should have been with the empty string instead of undef...thanks.