Learn English – Have existed vs. To have existed

This is the sort of 'rule' that teachers employ during early studies to prevent beginning students from making certain likely errors. In this case what the rule is fighting is things like:

• I never visited Italy, when what the student means is that she has not yet visited Italy
• I never saw Citizen Kane, when what the student means is that she is unfamiliar with the movie

This is the sort of proposition a beginning student is likely to encounter or use. But once the student masters the notion that past events (or non-events) with effects in the present are discussed in the present perfect form, the rule can be qualified: when you are speaking only of the past effects, the past form is appropriate:

• I lived in Europe in the 90s, but the entire time I never visited Italy.
• I never saw Citizen Kane until our school's Welles Festival last year.
• I never said I would give you my car. I said I would lend you my car.

ADDED:
To distinguish between use of the present perfect and use of the past:

I have never said I like hats means “Right up to the present moment I have avoided saying I like hats”; it suggests that you continue to avoid saying you like hats.

I never said I like hats means only that “During some [past] period [defined by the context] I avoided saying I liked hats”; it does not say anything about whether at some subsequent time you did say you liked hats, or whether you now deny or assert a liking for hats. It is silent on the topic.

It's been a long time since I have read an interesting book.

"Why is it not present perfect + past simple? I understand the "it's been" part, since it's something that is still relevant or going on. But if I'm referring to a specific past event, why should present perfect even be considered?"

I was reading your statement, and began thinking. And although Hellion is correct when he said:

"in both of the sample sentences, you're not referring to a specific past event, but to a somewhat generic experience; you can say "It's been a long time since I have read an interesting book", but you shouldn't say "It's been a long time since I have read The Hobbit".

He forgot a detail.

What if the last time I had read an interesting book was indeed at a specific past event, for example, last month? Hence a completed action, my reading an interesting book, at a definite time in the past.

Could a person then say:

"It's been a long time since I read (simple past) an interesting book."

I believe the answer is yes. It is grammatically correct and makes sense.

Here is another example taken verbatim from "A Practical English Grammar" (A.J.Thomson A.V.Martinet):

"I've worked here since I left school"

Which means I have finished attending school (for whatever reason but the most plausible being I have graduated or taken my final year exams) and now I am working i.e. I am not a student any more but currently an employee.