sub make_rand_str {
   my $num_chars = shift;
   my @possible_chr_array = @_;
   my $num_possibles = @possible_chr_array;
   my $rnd_str = "";
   for (my $i=$num_chars; $i--;) {
      $rnd_str .= $possible_chr_array[rand($num_possibles)];
   }
   return $rnd_str;
}

my $rnd_str = make_rand_str(4, "A".."Z", "0".."9");

my $url = "http://my/host/cgi/script.pl/$rnd_str",
[download]

Aside from the no-cache pragma, which I find works for 98%+ of folks out there, I find the simplest and easiest way to create a unique string is just a call to time(); e.g.,

$url = '/cgi-bin/script.pl?time=' . time();

Using both of these things in combination results in 99.9% effectiveness in preventing caching. (Estimates based on a scientific survey created by top scientists which was just recently pulled out of my *.)