What he has now is not a closure. It would be a closure if he used his idea of declaring the variable above and referencing it inside the sub.

UPDATE:
Actually, as long as he still passes by reference it's not a closure. Here's an example of what would make it a closure:

  my $ref_to_big_chunk;
  get_big_chunk(8675309);
  print "And the big data is: ";
  print $$ref_to_big_chunk;

  sub get_big_chunk
  {
    my $jenny_tel = $_[0];
    my $big_jenny = [ all work done here to make this huge ]
    $ref_to_big_chunk = \$big_jenny;
  }
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Note that this is a terrible way to do it because it's no longer obvious that the get_big_chunk() sub alters $ref_to_big_chunk.