my %domain_map;
$domain_map{'yahoo.com'} = ...default setting for all of yahoo.com...
$domain_map{'special.yahoo.com'} = ...special setting for special.yahoo.com...

sub find_setting {
  my $domain = shift;  # either from $ENV{HTTP_REFERER} or $ENV{REMOTE_HOST}
  my $setting;
  while (length($domain)) {
    defined($setting = $domain_map{$domain}) && last;
    $domain =~ s/^(.*?)(\.|\z)//;
  }
  return $setting;
}