This is an relative URL which give the location of the w3perl output directory from your webserver root. Don't give a path value !
W3Perl's output directory Web root directory Relative URL
/var/www/html/stats/ /var/www/html/ /stats/
/var/www/private/reports/web/ /var/www/ /private/reports/web/
/var/www/www-stats/web.com/ /var/www/web.com/ stats (*)

(*) if you are using a separate virtual host for w3perl's output (where you store reports for each server you're hosting for instance), you should create a symlink in each virtual host to the location of w3perl's output reports.

Example :
for your first server : webserver1
Ouput path is set to /var/www/www-stats/webserver1/
Your webserver1 root is /var/www/webserver1/
You should create a symlink from /var/www/webserver1/stats to /var/www/www-stats/webserver1/ (ln -s /var/www/www-stats/webserver1/ /var/www/webserver1/stats).
Linkpath_url should be then set to stats