In
form it is clearly a present tense, constructed with the present tense of HAVE; it contrasts with the
past perfect, constructed with the past tense of HAVE.
In
function, the present perfect appears to always have some connection with
both past and present. The situation referred to ocuurred, or at least began, in the past, and has some connection, in the speaker's mind, with the present.
As you rightly point out, a speaker of
AmE is more likely to say "Will just went out" while a speaker of
BrE is more likely to say "Will has just gone out." While there is no doubt that both speakers are referring to exactly the same situation, it is impossible to tell whether they see it in exactly the same way. It may be that the speaker of
AmE considers the word 'just' to be a past-time word (like, for example 'yesterday') whereas the speaker of
BrE considers it to be a word that refers to a past time so recent that it can be considered part of the present period.
I have never seen the present perfect used with no connection at all to to present time, though the connection is not infrequently implied rather than explicitly stated. I have
heard it so used, but only informally, in situations which suggest that the speaker has run two ideas together. Equally, I have never encountered a present perfect used exclusively with present reference.
"It has been soup." The present perfect is used in this joke precisely because it makes the pun possible. In real life "
it was soup, once" is probably a more likely utterance. However, the present pefect is not impossible - the speakers have the soup in front of them as they speak, the connection with the present. It would not be a natural utterance if the speaker were describing a bowl of something that had been served at a restaurant the previous day.