#!C:\perl\bin\perl.exe -w
use strict;
use Socket;
my $arg = shift @ARGV or die "Need a domain, dummy!\n";
my ( $domain ) = ( $arg =~ m!^(?:[^/]+/?/)?([^/]+)! );

print "Domain is " . get_domain( $domain );

sub get_domain {
    my @segment = reverse split /\./, shift;
    my $domain;
    return 0 if $segment[0] =~ /^(?:local|public)$/;
    SEARCH_FOR_DOMAIN: {
        foreach ( @segment ) {
            ( $domain = $_, next ) if not $domain;
            $domain = $_ . ".$domain";
            last SEARCH_FOR_DOMAIN if inet_aton( $domain );
        }
        return 0;
    }
    return $domain;
}