#!/usr/bin/perl -wT

use strict;
use CGI;
use URI;

use constant BAD_DOMAINS => qw(
  www.baddomain.com
);

#create lookup table
my %bad_domains;
undef @bad_domains{ BAD_DOMAINS() };

my $cgi = CGI->new;

print $cgi->redirect($cgi->referer)
  if exists $bad_domains{ URI->new($cgi->referer, 'http')->host };

print $cgi->header;

#...