Moshi moshi, Lady in
Japan,
I'm not 100% sure if I get your grammatical explanation.

You mean the subordinating conjunction "after" carries the "perfect mark", so that the
have -ed form can be omitted. I see "after" a subordinator as it is, and I would't think it has to carry the perfect meaning. Its main function and meaning is to indicate the order of two events. Here's my perception,
[You'll have time] after [they leave home.]
[An event] after [a general event.]
So If I am stating a general event at present, I'll just keep the present tense in the subordinating clause. Then, the event of the main clause, occuring after a general event, is considered to be happened at a point in future. So using a future tense would make perfect sense here.
What do you think? Cass?
All the best,
Blacknomi :-D