...
    # Encapsulated class data

    my %_attr_data; # for each package: stores reference to its attribute data only
    my %_mro_cache;
    my %_attr_data_cache; # for each package stores reference to its attribute data + inherited attribute data

    # Class methods to operate on encapsulated class data

    sub _get_attr_data
    {
        my $caller = shift;
        my $package = ref($caller) || $caller;
        return %{$_attr_data_cache{$package}} if defined $_attr_data_cache{$package};      
        my @parents = @{ $_mro_cache{$package} ||= mro::get_linear_isa($package) };
        my %return;
        for my $parent (@parents)
        {
            # assure that a reference is inside $_attr_data{$parent}
            ref $_attr_data{$parent} or next;
            for my $attr (keys %{$_attr_data{$parent}})
            {
                $return{$attr} ||= $_attr_data{$parent}{$attr};
            }
        }

        $_attr_data_cache{$package} = {%return};
        
        return %return;
    }
...