Very interesting observation, and this is actually documented in 5.8.1 perldelta.

They were saying that this is actually a feature in earlier versions, but some sort of lost in 5.6.1 (at least in 5.6.1, but also could be in other versions around that time frame). Now they got it back for us :-)

If you are interested in checking, go 5.8.1 perldelta, looking for "can't declare".