If your global variable is going to be used in two different requests (item-ae and append-item) then you can't share the data with a tcl variable. Not only because the two requests don't share this sort of state, but if two users are trying to work at the same time they would almost certainly clobber the other's idea of the correct variable.
You could either persist the item_id_list to your database, or have the client store the data (either through cookies or client-side javascript variables) until you're ready to persist it.
