Great article, Su.Ki.! I actually used to teach this kind of “British grammar” at inlingua. The difference in meaning would still be understood in American English, although, sadly, we don’t use “needn’t have” very much these days at all.

Hanji: That’s a great insight about the infinitive “to do” versus the perfect (bare) infinitive “have done”. I had always assumed “have done” was used because “need” is modal in that structure, but your theory makes perfect sense. The use of the perfect aspect alone could explain the difference in meaning. In fact, we get approximately the same effect without using “need” as a modal, although it doesn’t sound as smooth as “you needn’t have done that”:

“You didn’t need to have done that.”

Thank you for all the responses.

@Jack: I'm glad that you found this useful.

@ Andrea: Thanks for translating. Interesting, and very logical. I wish that we did more of this on italki!

@ Phil: Maybe here's a chance to answer your question. Let's find out:

@ Adam: Thanks for your insight. Can you tell us something, please? If you heard a 'fancy guy or girl' saying "I needn't have washed the dishes", would it be clear to you that they had actually washed the dishes? Or might you think that they were talking about Scenario 2? There's also a short question below, if you have time to take a look at that, too.

@ InverseImpact: Thank you for giving us your view on this. Can I ask you to answer the same question, please? Also, could you tell us what you think about the question below? Thanks.

Look at this dialogue and answer the question below.

Yes, that’s what I’m saying, although I suppose the younger generations could surprise me. Last I checked, American children were still required to read literature in English for school. If we go back a century, the “needn’t have” construction was common enough in American English. A lot of literature is actually from the UK, and if we go back a couple of centuries (which they still do in school, don’t they?), the USA didn’t even exist yet. On the other hand, if all that today’s teens read is Facebook and Twitter, who knows…. But surely that cannot be the case, can it?

You're welcome, Hanji. I'm glad you found it useful.

Thank you for sharing it. I've learned American English since I was a junior high school student so your post is a big present across the sea to the edge of the world where I live. I've noticed the difference between 'to do' and 'have done' in the two sentences, thanks to your detailed explanation.