(Thanks to Tanktalus for pointing out the relevant line of code :)

-command=>[\&_save_defaults($self)]
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means that you're calling _save_defaults passing $self, taking a reference to the result and putting it inside the anonimous array. This is why you're seeing the unexpected call. You'd better:

-command=>[\&_save_defaults, $self]
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or follow Tanktalus' suggestion (without the square brackets, you don't need if you pass the sub reference alone).