I tried your code with a small modification, and it worked. The second parameter to find() should be an array, so I modified it to this:

find (
  {
    wanted      => \&process,
    preprocess  => \&preprocess, 
    postprocess => \&postprocess
  }, 
  (".")
);
[download]

Update:

I understood your question, and the print out did show me that all three handler were executed. Hm...