$referer = $ENV{'HTTP_REFERER'};

my @valid_referers = ( 'http://www.yoursite.com/cgi-bin/a.cgi',
                       'http://www.yoursite.com/cgi-bin/b.cgi' );

print $query->redirect('/cgi-bin/errorpage.cgi')
    unless ( grep $_ eq $referer, @valid_referers );