I can suggest 2 possible solutions, both assuming that you have 2 separate fields - Month and PerDesc
1. Using Actions. You can use "OnSelect" trigger for the field PerDesc, and enforse a selection in the field Month every time the user selects or changes PerDesc. You may need to tinker with the field format, but generally, this is the direction.
2. Using a data modelling technique that I'm calling "as of date", where you create a linking table for the two Month Fields, and you associate a number of Months with each corresponding PerDesc. So, for example, if clicking on March needs to trigger a selection of Jan, Feb and March, then in your table you need to associate Months Jan, Feb and March with PerDesc = March.
In this case, when PerDesc Mar gets selected, all three associated months will be "selected" (or rather associated with your selection).
My personal preference is the second solution because it's using QlikView associative logic and not triggers and actions and other "pyrotechnics".