Is there any way for me to tell the script which domain to look at? Like the one that makes the call?

No.

If it is the "wrong" domain then the script won't even receive the cookie.

The client's browser won't send it. This is out of your control.

Well, what about having a script called check-cookie.pl at every domain? The user clicks a link to go to www.mymaindomain/cgi-bin/comments.pl and that somehow passes off to myfirstdomain/cgi-bin/check-cookie.pl which would include this:

local(@getcookies) = split (/; /,$ENV{'HTTP_COOKIE'});
local(%cookies);
   foreach(@getcookies){
    ($ckey, $cval) = split (/=/,$_);
    $cookies{$ckey} = $cval;
   } #end foreach(@getcookies)

$user_sid = $cookies{'phpbb2mysql_sid'};
$user_logged = $cookies{'phpbb2mysql_data'};
$user_lang = $cookies{'lang'};

if ($user_logged !~ /autologin/) {
print "$user_logged";
    &error(not_logged);
}
[download]

That would check the local cookie on that domain before passing back to the www.mymaindomain/cgi-bin/comments.pl script again. Shouldn't that work?

My only problem now is, how do I pass it off? I have a require line in the comments.pl that takes it to the check-cookie.pl script at myfirstdomain.com, but it doesn't read the cookie.