Um, in both examples you are not setting $_---you are only modifying it.
Interesting distinction. I think of them as synonyms, and would certainly argue that any statement that starts with "$_ =" is "setting" the variable. However, what name we use when giving a variable a new value is not really the point.
What makes both of these examples useful is that they implicitly alias $_ to another variable before acting on it. That's where they differ from the OP's usage.