When I read "Each time a block with a 'my' declaration is entered, a new instance of that lexical is created", my first reaction was very much "I think that's quite inaccurate".

I also quite disagree with the assertion that "it has no user-visible effects" and not just because you demonstrated some in your reply. The only reason I knew that the original description was inaccurate was because I had answered nodes from users who ran into things (which means they were user-visible) that, in order to explain, I had to make those specific effects more obviously visible.