Got something up and working, sort of, looks like, maybe
Added two lines at the beginning of the "open (GUEST)" section:
Old
foreach $line (@lines) {
if ($line =~ /<!--begin-->/) {
New
foreach $line (@lines) {
if ($INPUT{'url'}) {
exit;
}
if ($INPUT{'website'}) {
exit;
}
if ($line =~ /<!--begin-->/) {
As result, if in the new form anything is entered in the fields
name="url" or
name="website", that returns an Internal Server Error

, and nobody is the wiser why. The fields are not hidden but in plain sight, with at the beginning of the form a remark not to fill those fields. Unfortunately, I do not get info on who was hammering at my door through the file, but I worry about that later.
As extra, I remade the guestbook.htm and sign.htm in what.php and where.php, and renamed the functional guest.cgi script as well

. I left copies of the old files with the old names, still mentioned in the index.php, but not linked or visible on the site.
Hopefully, the bots will spent their time on the old guestbook routine (which nobody ever sees) and not on the protected one.
Question: I also have a bot-trap installed. Why do these bots not fall in the trap

but go for the guestbook? Very smart ones?