Consider theses two sentences:
Once I have graduated, I will find a job.
Once I graduate, I will find a job.
What's the difference between them?
Of all the resources about the present perfect I've found on the Internet, none talks about using it for a future action, but obviously, I see this use very often in English.
The present perfect describes something that happened at an uspecified time and when the exact moment of occurrence isn't important.
Although this usually means "an uspecified time before the present", the present perfect is also used in subordinate clauses to talk about an uspecified time in the future (when the main verb is in the present or future tense.)
This usage of the present perfect often follows these words and expressions:
I'll go shopping when the rain has stopped.
After the rain has stopped, I mow the lawn
I'll go shopping once the rain has stopped.
I'll go shopping if the rain has stopped.
Until the rain has stopped, I won't go shopping.
I make nachos before the game has started.
I'll stop playing as soon as this quarter has ended.
Note that we can also (and more frequently do) use the present simple to describe the future in these subordinate clauses with no change in meaning.
I'll go shopping when the rain stops.
After the rain stops, I mow the lawn.
I'll go shopping once the rain stops.
I'll go shopping if the rain stops.
Until the rain stops, I won't go shopping .
I make nachos before the game starts.
I'll stop playing as soon as this quarter ends.
There's really little functional difference between them. The use of the progressive tense always implies some kind of temporal relationship, but (I think) with instantaneous events it's a "difference that makes no difference".
"Finished" or "completed" can be similar:
I'll clean my room once I've finished breakfast.
I'll clean my room once I finish breakfast.
Only once Luke has completed his training, will he be ready to face the Emperor.
Only once Luke completes his training, will he be ready to face the Emperor.
As well as other "instantaneous" verbs like "pass/fail".
Once you've passed History, we can talk about summer camp
One you pass History, we can talk about summer camp.
I'm sure you can think of others.
There is no real difference. Usually, present perfect tense suggests that the event happens only once (or discontinuously), whereas present tense suggests that the event happens regularly (or continuously, depending on the context).
But here, using "once" in the sentence suggests the former. So it makes no difference whether you use present tense or present perfect tense.
