Your posted code runs fine for me, and prints "3" as the parent's test attribute value.

Output:

going in
in new for cChild and just calling sub_passed_to_child
yes in passed_sub
$oParent_ref is cParent=HASH(0xf18038)
$oParent_ref->{test_att} is 3
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So - you are doing it right !

If you wanted to pass the CHILD, instead of the parent at:

     ##OLD  $self->{sub_passed_to_child}( $self->{parent_obj_ref} );
          $self->{sub_passed_to_child}->( $self ); ##New, explicitly d
+e-ref the sub
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Then passed_sub would look like:

   sub passed_sub {
    print "yes in passed_sub\n";
    my ($child) = @_; # I don't like using shift here..
    print $child->{parent_obj_ref}{test_att}; # 3
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Of course, it would be "better", to use a "getter", so typos would be checked at compile time. </c>