As others have pointed out, you can't make it impossible, but you can try to make it hard. One of the entries in the Obfuscated Perl Competition a year or two back was an automatic obfuscation engine. The people running the competition commented that its output beat their attempts at automated deobfuscation. Something along those lines might be attempted.

However, it might be just as effective to use lawyers as your copy protection mechanism.

 --jonadab