Why do people say: "When I was little, I needed my soother to fall asleep." instead of: "When I was little, I needed my soother to have fallen asleep."? Isn't the second sentence the only correct one because of the sequence of tenses (consecutio temporum)?

The second one sounds wrong. The first one sounds correct. As an American, I have no idea what a soother is.

Needed sets the tense, the verb to follow should be an infinitive. Verbs of desire need to be followed by an infinitive.

In the second one it sounds like the falling asleep would happen before the use of the soother but that is logically not the case

First sentence is correct. Also, by soother I assume you mean pacifier, which is what most Americans would know it as.

What is a soother? I've never heard anyone say either of these things (because I've never heard that word). \[US\] The first sentence if correct. The second sentence (if it can be grammatically correct) is weird and has some very odd complex meaning that I can't really imagine. I think time travel would likely have to be involved.

"When I was little, I needed my soother to have fallen asleep" would actually mean that the soother is the thing falling asleep, and it would be incomplete grammatically - it would need a phrase at the end, like "before I could fall asleep too." It's extremely unlike that you'd ever want your soother/pacifier/dummy to fall asleep. Needed is already in past tense, so your wish for consecutio temporum has already been met. "Need to" is always followed by the infinitive.

"Have" is a verbal auxiliary marking the retrospective aspect (or "perfect"). It shifts the focus of a verb to its consequences; typically using the verb as justification for a state at the time of reference. "Why didn't you come here?" "I had fallen asleep" (so I was asleep at that time) So "I needed my soother to have fallen asleep" sounds strange because "to have fallen asleep" implies that you were already sleeping when you need a soother to sleep. It's contradictory in a sense; you're saying you needed help going to sleep, but were sleeping without it.

way too many comemnters are focusing on the soother part. but your question is about the verb conjugation. A clause that comes after the word "need" or "want" must be in the infinitive. it's similar to needing or wanting an object. for example - I need to study tomorrow. it's the same in the past tense. for example - when I was young, i needed to sleep. or - yesterday I needed to sleep.

This is an easy one to answer. Once you have fallen asleep, you don't need it any more. You need it **to** fall asleep. Not afterwards. You needed it *at the moment* when you *needed* to fall asleep. --- I needed money to buy food. ✅ I needed money to have bought food. ❌ - I'd already bought it, I no longer needed money.

No. And I have no idea what consecutio temporum is. To have fallen asleep with gum in her mouth, was stupid. To fall asleep when I was little, I needed my soother. — It was stupid of her to fall asleep with gum in her mouth. I needed my soother to fall asleep when I was little. Note (edit) Using the word “to” before a verb calls for the infinitive form of the verb (the root or base version). “Fall” in this case.

No, because infinitives don't really have tenses. You're absolutely correct that the perfect infinitive will sometimes be used to talk about past events. For example, you could say something like "She claims to have met..." instead of "She claims that she met...". But in the vast majority of cases, you still just use the "normal" infinitive

No, "to fall asleep" is an infinitive. Consider this sentence: "Yesterday, I wanted to play basketball" You wouldn't say  "Yesterday, I wanted to have played basketball" Same grammatical structure here. "I needed ..... to fall asleep". The first verb is conjugated to indicate tense, the second is used to describe the soother so it doesn't really have a temporal tense, it uses the infinitive.

I think the first one might make more sense if you rearrange it slightly, to "When I was little, in order to fall asleep, I needed my pacifier." The second one sounds like you need your pacifier to be sleeping which doesn't make any sense.

The first sentence is in past tense, the second is in past perfect tense, which we use for talking about things that had been completed before the time we're talking about in the past.

Something else nobody is mentioning... When you say "my soother (pacifier)" with the past tense, it gives the impression that you still own this item. If you use "a", it gets the message across that you used one, and the fact that it was yours can be implied. You probably don't use a pacifier as an adult. "When I was little, I needed a soother to fall asleep"

The first one is more natural in this case. I don’t often see the present perfect infinitive, if we can call it that, often, but it does happen in cases where the past action applies to the current situation. “Having memorized this song doesn’t mean understanding it” sounds the same as “memorizing this song […]” in the context of memorized something before the current conversation, but to me, there’s an emphasis on the act of completely memorizing. In this situation, it sounds wrong because “having fallen asleep” sounds like a one time action completed in the past relevant to now, but the context of “when I was little,” meaning over a period of time rather than one occurrence, makes it incorrect.