'poproszę' which has a perfective/future meaning, correct?
From a technical point of view, yes.
Poproszę is (simple) future tense per definition. But it's used as present tense. Call it an exception if you want.
Word by word translation. But it's not used with this translation:Proszę herbatę = I'm asking for tea
Poproszę herbatę = I will ask for tea [
The only practical difference is that poproszę is more polite and more common.
But both of them are always used to politely express that you want sth from the person you are talking to (to order/buy sth). It doesn't make sense technically. But we have to accept that.
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Used with verbs to express imperative we can only use
proszę (and it's formal).
Proszę wejść! (Come in!)
Proszę iść do lekarza! (Go to the doctor!)