If I understood correctly, you want Drall to be called with the current location whenever a file index.html isn't found. You can do as follow.

Replace the redirect to Drall with a call to a printenv script:

DirectoryIndex index.html /cgi-bin/printenv.pl

This way, whenever you visit a location without index.html, you get redirected to the script. If now you look at what printenv.pl says, you will see many headers prefixed by REDIRECT_, and the uri you were looking for is in REDIRECT_REQUEST_URI.

So, the solution is to substitute printenv.pl with a script which issues a redirect to Drall with the correct paramaters, something like

use CGI;
$redir = '/cgi-bin/drall.pl?left='. $ENV{REDIRECT_REQUEST_URI};
print CGI::redirect(-uri => $redir);
[download]

Hope this helps! Ciao, Valerio