You've misunderstood "preprocess". From the docs:

       "preprocess"
          The value should be a code reference. This code reference is
+ used to
          preprocess the current directory. The name of the currently 
+pro-
          cessed directory is in $File::Find::dir. Your preprocessing 
+function
          is called after "readdir()", but before the loop that calls 
+the
          "wanted()" function. It is called with a list of strings (ac
+tually
          file/directory names) and is expected to return a list of st
+rings.
          The code can be used to sort the file/directory names alphab
+eti-
          cally, numerically, or to filter out directory entries based
+ on
          their name alone. When follow or follow_fast are in effect, 
+"prepro-
          cess" is a no-op.
[download]

It's doing what you ask. Your preprocess is returning the single value "1" (the result of the print).