This is just an error in your logic, basically. What you want is probably:

    if ($File::Find::name =~ m!^/some_dir/sub_dir!) {
        print "skipping $dir\n';
        $File::Find::prune = 1;
        return;
    }
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In other words, test $File::Find::name, not $File::Find::dir. When process gets called, dir holds the name of the *containing* directory. So when process gets called for the entry "/some_dir/sub_dir", dir is "/some_dir". You're expecting it to be "/some_dir/sub_dir", but it's not that until you've actually descended into "sub_dir". So you're skipping all the entries below "sub_dir" but not descending into "sub_dir" itself.

Make sense?

By the way, "$dir/$_" is just $File::Find::name. I think.