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.
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:
print "And the big data is: ";
my $jenny_tel = $_;
my $big_jenny = [ all work done here to make this huge ]
$ref_to_big_chunk = \$big_jenny;
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.