Chained method calls make backwards compatibility hard to do, if you make an attribute into its own object at any time. You can't use Want.pm, because either way you want an object.
You mean, it makes it difficult to change a method such that has a different return value? Well, that always is non-backwards compatible. Whether the user chains, or stores the result in a variable and does something with the variable, it's going to be non-backwards compatible.
Chained method calls force a person reading to check whether a mutator has returned the same object or a more specific object.
And that's different from storing the result of a method (whether said method is a mutator or something else) in a variable, and call a method in said variable?