I recall being surprised that $_[0] was a shallow copy and that assigning to $_[0] didn't change (the scalar holding) the object that invoked the method. I recall this being discussed in a node here (a while ago). I recall being surprised to hear the opposite (more recently but still at least weeks, probably months ago).

So I think there used to be protections (perhaps unintentional) against modifying $_[0] in a method call and a while ago Perl changed in this regard.

Sorry, I was unable to find hard references in the time allotted. I hope the wild rumors prove of some use anyway.