It's wise to ask these questions on the forums. I hope everyone continues to do so. :-)
To the best of my knowledge no-one has written perl scripts to process this particular information. However check with Aaron Wangberg at Winona State University because he has mined other similar information from the WeBWork database.
More importantly consider looking at webwork version ww2.5.2 (currently a reasonably stable beta -- but definitely not released yet). The addition of the essayQuestion type by Geoff Goehle at Western Carolina University also replaces the answer_log by a database table that serves the same purpose. You might prefer to develop for the beta version of WeBWorK or at least be aware that your perl script might soon have to be adapted to accept input from a database instead of from a log file.
To get version ww2.5.2 look in github.com/openwebwork/webwork2-dev branch ww2.5.2.
See also http://webwork.maa.org/wiki/Release_notes_for_WeBWorK_2.5.2