In reply to verif_learner:
Your get will not be successful, because you are restricting the hierarchy by the set command.
The 3 set commands have the same result. Only a component in the hierachy top.module_a.instance_a can retrieve a value.
In your get you are considering only the hierarchy top.instance_a which does not meet the hierarchy from the set which is top.module_a.instance_a.