Because the object ID is stored in the object's scalar ref (i.e., $$self), overloading ${} deferencing is not allowed. I document this fact in the POD.

However, all other forms of object deferencing through overloading are supported: Stringify, numerify, boolify, arrayify, hashify, globify and codify. Object::InsideOut simplifies defining these operations through the use of subroutine attributes. For example, your code would become:

use Test::More 'no_plan';

package Foo::Bar; {
    use Object::InsideOut;

    sub numify :Numerify { 42 }
}

package main;

my $obj = Foo::Bar->new();

is(0 + $obj, 42);
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