use NetAddr::IP;
my $sub6 = new NetAddr::IP '2001:db8::/40';

my $i=1;
while ($sub6 < $sub6->broadcast) {  # should've been <= but it does not seem to work
        if ($i==4722366482869645213696) { # that many addresses in a /56
                print "ip = $sub6\n";
                $i=1;
        }
        $sub6 ++;
        $i++;
}
print "ip = $sub6\n";  # need to print last address