use strict;
use warnings;
use Data::Dump qw/pp dd/;

package Class1 {
    use constant cfg => { a => 1 };
};

package Class2 {
    use parent -norequire, 'Class1';
    use constant cfg => { %{__PACKAGE__->SUPER::cfg}, b => 2 };
};

package Class3 {
    use parent -norequire, 'Class2';
    use constant cfg => { %{__PACKAGE__->SUPER::cfg}, c => 3  };

    sub meth { warn "a is ",cfg->{a} }

};


pp
  Class1::cfg,
  Class2::cfg,
  Class3::cfg
  ;

Class3->meth();