Hello
Instead of using the perform statement it is better to use the programming exit to change the work item description dynamically
http://scn.sap.com/people/rj.vanlooy/blog/2012/01/29/dynamic-user-decisions-in-sap-business-workflow
you can also determine the text dynamically in background workflow step and store the results in workflow container elements and then use these container elements in task description
regards
sandy