How can it affect the order of the directory processing?

Its effect is that it changes what is passed to the wanted() coderef e.g

find({
    preprocess => sub { return sort { $b cmp $a } @_ },
    wanted     => \&process_array,
}, @ARGV);
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This will sort the directory listing that process_array() gets in reverse alphabetic order. So basically the preprocess option allows you to change what the wanted() function receives from changing it's order to removing unwanted directory entries.
HTH

_________
broquaint

update: changed <=> to cmp in the sort() (thanks ariels)