Perl Regex 获取 URL 的根域

我怎样才能获得网址的某些部分？

例如：

http://www.facebook.com/xxxxxxxxxxx
http://www.stackoverflow.com/yyyyyyyyyyyyyyyy

我只需要考虑这一部分：

facebook.com
stackoverflow.com

use feature qw( say state );

use Domain::PublicSuffix qw( );
use URI                  qw( );

# Returns "domain.tld" for "subdomain.domain.tld". 
# Handles multi-level TLDs such as ".co.uk".
sub root_domain {
   my ($domain) = @_;
   state $parser = Domain::PublicSuffix->new();
   return $parser->get_root_domain($domain);
}

# Accepts urls as strings and as URI objects.
sub url_root_domain {
   my ($abs_url) = @_;
   my $domain = URI->new($abs_url)->host();
   return root_domain($domain);
}

say url_root_domain('http://www.facebook.com/');       # facebook.com
say url_root_domain('https://www.facebook.com/');      # facebook.com
say url_root_domain('http://mobile.google.com/');      # google.com
say url_root_domain('http://www.theregister.co.uk/');  # theregister.co.uk
say url_root_domain('http://www.com/');                # www.com