Adding a client workspace spec. like jenkins-$JOB_NAME will cause a P4 error:

xxx $ "C:\Program Files\Perforce\p4.exe" -s client -i
Caught exception communicating with perforce.  Slashes (/) not allowed in  xxxxx

when used in a matrix project. Something like below:

String nameFixer(AbstractProject project) {
        String name = project.getFullName();
        return name.replace('/','-').replace('=','-').replace(',','-');
    }

when substituting "JOB_NAME" is a possible fix to create valid workspace name.