Making the var read-only won't prevent concatenation or addition, so it doesn't matter that $x . [] or $x + [] doesn't make sense.[1]. This isn't relevant to the topic at hand.
The only thing making it read-only will do is prevent assignment, and map { $_ = 3 } $x.$y makes no more sense than map { $_ = 3 } [].
Either prevent both of them, or prevent neither of them. No need for baseless inconsistencies.
- That said, $ref+0 makes some sense for refs to things other than objects with overloads, and both concatenation and addition make sense for refs to objects with overloads (which aren't farfetched at all!)