I've run across this sentence on a language learning forum, and it claims to be an example of perfect tense. It's been many years since I've had a formal grammar lesson, but this appears to me to be a disagreement of tense, between "would have given" (past) and "everything I have" (present). I do not see how in the past one could have given what one now possesses in the present. Am I simply misremembering this usage, or is this sentence in fact confused?
It isn't strictly the past, it is the conditional perfect.
It would have been equally grammatical to have said I would have given you everything I had - which you may find more acceptable.
But as @Fumble Fingers says, if the speaker still has what they had then, then have is fine - especially if it all happened in the recent past.
I know it as the subjunctive tense/mood, which is used to describe conditional situations as in the famous example:
If I were a rich man
In this case, "were" is the subjunctive of "to be," rather than the past tense. In this conditional sense, he could be a rich man in the past, present, or future, since whatever follows the condition holds true in any time frame as long as the condition is fulfilled, unless otherwise specified in the text.