There are two different XML modules used by the CESM scripts. One is XML "Lite", which ships with CESM, and the other is LibXML, which should come with your Perl installation. The problem you are having is from in ConfigCase.pm, which uses LibXML, so make sure that the file "LibXML.pm" is installed on your machine and can be read from the compute nodes (where the job is running) as well as from the login nodes (where you are entering commands).