use strict;
use warnings;
use DBI;
use CGI;
use CGI::Carp qw(fatalsToBrowser);

my $cgi   = CGI->new(); 
my $sid   = $cgi->cookie('login');
my $login = 'http://localhost/cgi-bin/login.pl';

if ( ! $sid ){ # exit and return to login.pl! we have no cookies 
    print $cgi->redirect($login);
}

#start using the page! we have cookies
print $cgi->header();

# etc