sub recurse_inorder {
  my $self=shift;
  my @inorder;
  my $sub;
  $sub=sub{ 
            my $node=shift;
            $sub->($node->left) if $node->left;
            push @inorder,$node->value;
            $sub->($node->right) if $node->right;
          };
  return \@inorder;
}
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Maybe I'm just dense, but when does the actual population of @inorder happen?

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