First, a very simple example of a closure:

{
  my $tree;

  sub get_tree {
    return $tree;
  };
};
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In the above example, $tree and all memory referenced by it will never go out of scope, and since $tree is invisible outside of its containing block, there is no way to free up the referenced memory.

{
  my $tree;

  sub get_tree {
    return $tree;
  };

  sub release_tree {
    undef $tree;
  };
};
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In this example, it is possible to release the memory structure referenced by $tree by calling release_tree, because it also sees the $tree variable. But this release is not automatic and thus must be done by the programmer when the time has come.