Would it be possible to start with the last two sections of the domain, and then add one section at a time until Whois gives a result? For example, try 'co.uk', which wouldn't work, then 'lexicon.co.uk', which would. Something like this: 
my $host = 'www.lexicon.co.uk';
my @parts = split /\./, $host;

my $domain = pop @parts;
my $whois = '';

while (@parts and not $whois) {
  $domain = pop(@parts) . ".$domain";

  $whois = Whois($domain);
}

print $whois;

[download]
With adjustments for the specific Whois module being used, of course.

In reply to Re: Extracting domain names from FQDNs by chipmunk
in thread Extracting domain names from FQDNs by Ovid

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