> Would you be willing to provide an example where this becomes a problem?

It's not a specific example, as much as every piece of code where mutability is involved. I don't have a LOC number in mind, but it could be for blocks/local scopes. IMO syntactic explicitness helps clarity and thus bug hunting/maintenance, and I feel tooling shouldn't be an alternative for dealing with, albeit subjective, syntactic shortcomings.

    > this should be a problem for structured applications and hints at a more fundamental design problem

Frankly, I don't see how introducing an explicit hint that a variable is created can ever be a bad thing. Explicit lack of mutability is also something I'd like to have (e.g. 'let' vs 'var') - sometimes I want to make sure not to accidentally change a value. We will shoot ourselves in the foot at some point, so a syntax that allows for a check whether I can actually change a value further down can only be a good thing.

I absolutely do not trust myself. :)

When things go wrong for this reason (difficult to spot typo etc) I may waste time on investigating whether blocks are actually leaky or not etc - I have issues with Python syntax for the same reason. Going in the other direction, I like Rust's perhaps overly verbose "let mut" much better, since this has other consequences for explicitness (mutable references '&mut X' vs immutable references '&X'). No special tooling or IDE required to spot this, I can 'cat' the code in a terminal and it's still obvious.

I absolutely don't think Julia or Python should "go Rust", since their dynamic nature is a strength, especially so in academia where notebooks are popular. But a simple "I'm creating a variable" should IMO be a nice visual cue to any developer.

It comes up all the time 100 line one-off scripts in python. You start off with 10 lines that you're iterating on. You move some snippet to a function, don't de-dupe your variable names from the global scope, and you get a bug where you thought "x" was scoped to the function, but wasn't actually defined in the function so you're using the global "x". Yes, this could be fixed by moving the global statements into a main() function, but this breaks your repl workflow and you have to resort to breakpoints in main().