Preventing script from being called by other domain

Hi,

I don't write perl. I just configure prebuilt scripts.  I need to add a line of code that will prevent the script from being called by any other domain other than the domain that the script is installed on.

Here is a copy and paste of  the current script.  Can you insert the necessary line(s) of code?

For example, this script is installed at www.foobar.com (domain example has been changed from the actual domain for privacy purposes.)


Thanks!
============

$|++;
use Net::SMTP;
use CGI;
$q = new CGI;

$foobarfolks='<jjones@foobar.com>, <jsmith@foobar.com>';
$toddemail='<jones@foobar.com>';
$message="Thank you for contacting US.;
$user_subject='Thank You from US';

foreach ($q->param){
    $$_=$q->param($_);

    next if /^to$/;
    $$_=~ s/^\s+//g;
    $$_=~ s/\s+$//g;
}

if (!($to)){
    print "Content-type: text/html\n\n";
    print "Please go back to the form and enter your email address.<BR>\n";
    exit;
}

if (!($firstname)){
    print "Content-type: text/html\n\n";
    print "Please go back to the form and enter your First Name.<BR>\n";
    exit;
}

if (!($lastname)){
    print "Content-type: text/html\n\n";
    print "Please go back to the form and enter your Last Name.<BR>\n";
    exit;
}

@new_from=split(/,\s/,join(', ',$from,$foobarfolks)); ## NOTE the , inside the single quotes

if (!($to)){
    print "Content-type: text/html\n\n";
    print "Please go back and enter your email address<BR>\n";
    exit;
}

#     Email the user who posted the form, a TY note!!
$smtp = Net::SMTP->new('localhost'); # connect to an SMTP server
$smtp->mail($toddemail);     ##     use the sender's address here
$smtp->to($to);     ##     recipient's address
$smtp->data();
$smtp->datasend("Subject: ".$user_subject."\n");
$smtp->datasend("\n");
$smtp->datasend($message);


$smtp->dataend();                   # Finish sending the mail

##  Close the connection
$smtp->quit;

foreach $new_from (@new_from){
    $smtp = Net::SMTP->new('localhost'); # connect to an SMTP server
    $smtp->mail($to);     ##     use sender's address here
    $smtp->to($new_from);     ##     recipient's address. Use new_from, not from
    $smtp->data();
    $smtp->datasend("To: ".$new_from."\n");
    $smtp->datasend("Subject: ".$your_subject."\n");
    $smtp->datasend("Copies of this email sent to: $foobarfolks \n\n");
    $smtp->datasend("\n");
    $smtp->datasend("Name : $name\n");
    $smtp->datasend("Email : $to\n");
    $smtp->datasend("Home Phone : $homephone\n");
    $smtp->datasend("Work PHone : $workphone\n");
    $smtp->datasend("Referer     : " . $q->referer . "\n");
    $smtp->datasend("Remote Host : " . $q->remote_host . "\n");
    $smtp->datasend("User Agent  : " . $q->user_agent . "\n");
     
    $smtp->dataend();                   # Finish sending the mail
   
    ##  Close the connection
    $smtp->quit;
}


##     Redirect to the Thank you page.
print "Location: https://www.foobar.com/html/content/response1.shtml\n\n";