For large perl/Tk projects, I usually put all the gui code for each window or dialog into it own _ui routine. So for your project, I would use notebook_ui, tab1_iu, etc. The helper routines for each _ui would be in proximity to the _ui sub.

As I project grows, I move each _ui + helper subs into it's own module, for ease of locating subs. Creating a namespace for each module is a good idea if you think there will be a lot of collisions among sub names; otherwise, don't bother with it and just disambiguate sub names.

Unless you have a whole lot of interaction between different tabs' subs, this should be pretty painless to do incrementally.