In addition to the answers you'll get regarding caller, also consider that with the right .htaccess directives, you can also make a URL illegal to fetch, returning a 404 or 403 error. For example:

<Files *.pl>
order allow,deny
deny from all
</Files>
[download]

in an .htaccess file denies all access to .pl files. (I'm doing this from memory, but the syntax is at least similar to that.) </code>