Try with:

    if ($uid < 500){
        print "UID is $uid, skipping $File::Find::dir $_\n";
        $File::Find::prune = 1;
        return;
    }
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Setting $File::Find::prune global to a true value, you are telling find() function to avoid recursing into the current dir. You can find a (brief) mention of this feature on File::Find documentation