As Aristotle says, ref $_[0] eq "PKG" will break inheritance.

I think that illustrates a serious problem with the concept. I took the stance that:
Class::derived( $not_blessed); should do the same as:
$class->derived( $not_blessed); and decided that inheritance was not allowed because the first form was desired.

If allowing naive users to use the various calling conventions
was the idea, and why else, I thought this was the clear choice.

The following, and more, need to be considered:

Class::derived( $class_obj);             # Jack
Class::derived( Class $not_blessed);     # Queen
Class->derived( $not_blessed);           # King
$class_obj->derived( $not_blessed);      # Ace
Class::derived( "Class", $not_blessed);  # Joker
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The last is the joker. How will you distinguish between the Joker and the Queen?
And what about Jack? Is that the same as:  $class_obj->derived;
or the same as:  $class_obj->derived( $class_obj);