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push @{ $parent->{children} }, $child;
weaken( $child->{parent} = $parent );   # Needed to avoid memory leak.

{
   Resource res = get_resource_exclusively(res_id);
...
   // ...
   res.release();
}

Resource res = get_resource_exclusively(res_id);
try {
...
   res.release();
   throw e;
}