It's a very wide spread myth that objects are blessed references. In fact, they are references to blessed variables. The difference? The thing that is blessed.

I agree that the syntax of bless is misleading. But if you bless \$foo, it is $foo that is actually blessed. This means that you can choose to return \$foo; and it'll still work, even though the scalar assigned for the second reference you're creating is an entirely different one. It's not the reference, but $foo that is blessed.

@_ is an array of aliases. These are aliases for the variables used in the call. When such a value/variable is mutable, the element in @_ is automatically as mutable, because it's the same thing. This is a danger with using any element of @_ directly, not just when they're overloaded, and not just with bless. Even assignment to one of the elements is dangerous. The lesson is simple: if you want to change something without changing it, copy and change the copy, so the original stays intact.