Using the WHM you are able to install Perl modules via the interface as shown here. However, sometimes modules may fail to install for various reasons, either missing dependencies or other problems, and to gather more information about the failure, it is necessary to perform the steps via the command-line.
cPanel provides a wrapper-script for CPAN that can be used to install Perl modules.
To install a Perl module simply specify the name of the module and the script will handle the rest. If any failures are reported during the installation you will also have a log and more verbose output to work with.