Are there any odd side effects of that blessing?

I think yes.

Firstly, if someone uses the string value of an unblessed reference as a hash key (for example for memoizing some function of the reference), then calling refaddr on this reference would change the string and thus make the program fail. That's a bad practice anyway, as string values of scalar references can change even without reblessing, as the prefix can be SCALAR, REF, GLOB etc. You should use the numeric values of references instead. However, some code can still use it, do I'd consider this an odd side effect.

Secondly, doesn't this function blow up on a reference to a constant, such as $x = \5; $ad = refaddr($x);? It still makes sense to take the address of such a reference.