my @friends = (
  'www.mysite.com',
  'www.yoursite.com',
  'www.hissite.com/mypage/'
  'www.hersite.com/herpage/'
);

foreach $site (@friends) {
  if ($ENV{'HTTP_REFERER'} =~ m/$site/i) {
    $isafriend = 1;
  }
}

if (!$isafriend) {
  # do the stuff here, Location, html, etc.
}