If I remember correctly, Matt's Script Archive had another approach.

He also uses $ENV{'HTTP_REFERER'} but instead of comparing to 1 URL, he sets a few @friends. Something along the lines of

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.
}
[download]

Although this provides a way to compare a few sites, there is a major security flaw (that I feel can be easily fixed) by allowing just $ENV{HTTP_REFERER} to match 'www.mysite.com'... You see, I might as well have an URL like "http://badsite.com/www.mysite.com/" and it would pass!

Am I rambling already? Ok, I'll shutup.

#!/home/bbq/bin/perl
# Trust no1!