use v5.12;
use strict;
use warnings;
use MRO::Compat;

package My::Base {
	my %_attr_data;
	my %_mro_cache;
	sub _get_attr_data {
		my $package = shift;
		my @parents = @{ $_mro_cache{$package} ||= mro::get_linear_isa($package) };
		my %return;
		for my $parent (@parents) {
			ref $_attr_data{$parent} or next;
			for my $attr (keys %{$_attr_data{$parent}}) {
				$return{$attr} ||= $_attr_data{$parent}{$attr};
			}
		}
		return %return;
	}
	sub _set_addr_data {
		my $package = shift;
		$package eq caller or die;
		$_attr_data{$package} = ref($_[0]) ? $_[0] : {@_};
	}
	__PACKAGE__->_set_addr_data(
		'_root_entry1' => [ undef, 'read/write'],
		'_root_entry2' => [ undef, 'read'],
	);
}

package My::Derived {
	use parent -norequire, 'My::Base';
	__PACKAGE__->_set_addr_data(
		'__some_entry1' => [ undef, 'read' ],
	);
}

package My::Derived::Derived {
	use parent -norequire, 'My::Derived';
	__PACKAGE__->_set_addr_data(
		'__some_entry1' => [ 123, 'read/write' ],
		'__some_entry2' => [ 456, 'read/write' ],
	);
}

use Data::Dumper;
print Dumper { My::Derived::Derived->_get_attr_data };