edit RAD_REQUEST_PROXY_REPLY with perl

Hi, I'm trying to edit the values in RAD_REQUEST_PROXY_REPLY with perl. I've started with the example.pl file, and I'm able to list the first different attributes but I'm not sure how to list all the values, and how to change them. Specifically I want to edit the values of cisco-avpair attributes (which are multiple), replacing specific strings. I don't want to add a new attribute, but to change one or more of the attributes before they are sent to my NAS. Can anyone help me with an example of how to traverse and change these? (btw, I'm using freeradius version 2.1.3) Thanks, Tim