I left the setting "Preserve array formatting" unchecked and it breaks the callback function. My suspicion is that it won't break if I enable the setting but I'm not sure.
   // Look through the child items for the MENU_DEFAULT_LOCAL_TASK.
-  $child_paths = array_filter(array_keys($items), function($p) use ($path) {
-    return preg_match("#^$path/[^/]*$#", $p);
+  $child_paths = array_filter(array_keys($items), ($p) ($path){
+    preg_match("#^$path/[^/]*$#", $p)
   });