Use CGI.pm for grabbing the parameters, and read up on Taint mode for protecting against such attacks. A search for 'Taint' on this site should yield lots of good results.
After I replied I looked at what this code actually does
... you must be very cautious. You are allowing some very dangerous things to be specified by the user, such as paths to files. This approach invites grave danger to the security of your server.
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