New Approach
Routing:
In some instances, however, you may want to remap this relationship so
that a different class/method can be called instead of the one
corresponding to the URL.
A short excerpt from the CI doc shows that you can't use routing here. Instead you should go for a mod_rewrite rule (.htaccess)
RewriteEngine On
RewriteRule ^index/Cpanel_control/(.*) Cpanel/$1 [R]
Old approach
According to the corresponding documentation, the paths are both not absolute. Furthermore, you need to set the "from"-URI as the array key and the "to" URI as the string. If you want to route index/Cpanel_control to Cpanel, you need to swap the URIs of you example.
So this would be correct:
$route['index/Cpanel_control'] = "Cpanel";