Your code seemingly intentionally finds something other than the TLD in some situations.

• http://www.ibm.com/ ⇒ com (ok)
• http://www.ibm.ca/ ⇒ ibm.ca (not tld)
• http://www.ibm.co.uk/ ⇒ co.uk (not tld)

It also mishandles a number of the valid urls listed below. Fix:

$url = URI->new($url);

defined( my $host = $url->host() )
   or die("No host\n");

my $tld;
if ($host =~ /\./) {
   $tld = /\.([^.]+)$/;
   $tld =~ /[a-z]/i
      or die("No domain\n");
} else {
   $host =~ /[a-z]/i
      or die("No domain\n");
   $tld = 'localdomain';
}
[download]

Handles valid urls

• http://example.com/ (com)
• http://example.com./ (com)
• http://example.com (com)
• http://example.com:80/ (com)
• http://example/ (localdomain)
• http://www.ibm.com/ (com)
• http://www.ibm.ca/ (ca)
• http://www.ibm.co.uk/ (uk)
• http://www.ibm.com.au/ (au)
• http://192.168.0.1/ (error)
• http://3232235521/ (error)

Invalid urls aren't necessarily detected.

Update: Updated code to detects an invalid url it didn't detect before.