I thought that the use of

use Object::InsideOut qw(My::Parent);
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was only for classes of the type Object::InsideOut. And I wanted a regular class (eventually hash based). But it works! Thanks.

This is a drawback, imho, of using Object::InsideOut. For example, if you put a class in the cpan, based in the Object::InsideOut, the use of the class it's different from the expected.