He is saying that it would make it impossible to do things like (in spirit)
local $foo->bar = 'baz';
ie dynamic scoping. If the sub was responsible for the actual assignment, this would be impossible because it can't know whether the assignment is scoped dynamically and because there is no obvious mechnism for undoing the assignment at the end of the block. If you require validating to go through a tied proxy object, all of this is trivial since all the existing language hooks work as expected.
Makeshifts last the longest.