Unfortunately, a command like mvexpand does not exist for eval. However, to workaround this issue, you can use the eval commands to join the field values together as one value, run the urldecode function, and then split the values back in to a multivalue field. An example for how to do this is:
EVAL-myURL = split(urldecode(mvjoin(myURL, "&")),"&")
Bear in mind that depending on your data, you may need to use a different delimiter to join and split the field. Using the previous example, it might not be desired if your URL contained ampersand characters such as
http://www.splunk.com?answers=1&lang=en
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