What is the difference in meaning between these two sentences?
1: You can not know how much I have written on a piece of paper
2: You can not know how much I wrote on a piece of paper
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1: You cannot know how much I have written on a piece of paper.
The present perfect tense refers to an action or state that either occurred at an indefinite time in the past or began in the past and continues in the present time.
Your sentence indicates that the action occurred at an indefinite time in the past, since there is no particular time period mentioned. It can also happen that the writer is still in possession of that piece of paper, indicating present tense of the action.
2: You cannot know how much I wrote on a piece of paper.
The simple past tense shows that you are talking about something that has already happened. The simple past tense emphasizes that the action is finished.
The sentence simply denotes the action is already in the past now.
As for your question, difference in meaning, there is no such. The only thing we can be sure about is the fact that the action happened in the past. As pointed out by Colin in the comments to my answer, which I am thankful for, is whether the writer or the speaker includes the present relevance in his speech or not. This reflects that the difference between the past and the perfect is not actually a difference of tense, but one of aspect, of speech and hearing.