Hmm, the solution is correct but, in my view, the explanation seems to miss the main point:

I don't think so Laurent_R, that is impossible
The error message says Can't use string(".") as an ARRAY ref
My answer title says (to create an arrayref use [ ] )
My answer says split returns a list, to make an array you need [ 'square' , 'brackets' ]

your view is tiny

split returns a list, to make an array you need 'square' , 'brackets'

Well, this:

(split /$separator/, $line)
[download]

gives you an array, on which you can use an index to find one element, for example:

my $third_elmnt = (split /$separator/, $line)[2];
[download]

On the other hand, this:

[split /$separator/, $line];
[download]

does not give you an array, but an arrayref i.e. a single scalar. And an arrayref is what you need to oush onto an array if you want to build an AoA.

You probably know all that, I was only objecting to the fact that this was not made very clear in my humble opinion.

Actually, there is a subtle difference between lists and arrays in Perl. Functions like split return lists, not arrays. See What is the difference between a list and an array?

gives you an array, on which you can use an index to find one element, for example:

No, a list is a list and not an array, perl lets you index a list, it doesn't make a list an array

On the other hand, this: does not give you an array, but an arrayref i.e. a single scalar. And an arrayref is what you need to oush onto an array if you want to build an AoA.

When you make an arrayref, what is that reference pointing to?
An array right?
so creating an arrayref creates an array (it make an array )

You probably know all that, I was only objecting to the fact that this was not made very clear in my humble opinion.

Well, if you wrote something like:
to clarify
to elaborate
to make more clear
to make more clear some details
to make more clear some subtle details
... some kind of words other than ... miss the main point

I probably wouldn't have responded

Correct corrections are welcome ; elaborations are usually welcome (if they don't go off topic from main thread too much)

wrong corrections not so much; they call the duty