backup.pl is not used. What ISPmanager version you use? ISPmanager run /usr/local/ispmgr/sbin/pbackup and use /root/.backup.conf for configuration and this must be configured in ISPmanager's backup module
http://en.ispdoc.com/index.php/Backu...t_(ISPmanager)
You should check /usr/local/ispmgr/var/pbackup.log