What's the difference between the two? Is the latter more formal? Example:
As I (have) told you before, I didn't know the victim.
Both have many results in Google Books: As I told you before. As I have told you before.
So I'm sure they're both grammatically correct.
Both versions are grammatical, and the difference between them is pretty subtle. (It's hard for me to imagine a situation where one would be correct, but the other not.)
To my ear, as I told you refers to one or more specific instances of my telling you. I would use it if I were emphasizing that I had, for instance, upheld my responsibility to inform you of something.
The perfect tense, as I have told you, on the other hand, is used to allude to the consequences or implications of past action, and doesn't require me to have a particular time in mind. In fact, it would be ungrammatical to say, #as I have told you yesterday, ....
I would use as I have told you if I were emphasizing that you should already know the thing I told you.
I agree with the above from a grammatical point of view, which doesn't mean one would not find the sentence "I have told you yesterday" in the spoken and written language.
