It could only be 1 or 4. An empty hashref that's not at the end of a block, file or list or in a complex expression should be terminated by a semi-colon (like any other (sub)expression). A block is terminated by its closing brace.

My main objection is that it's silly to have a discrepancy between the current behaviour:

{} # hashref
[download]

and the proposed:

{}{} # two blocks...
[download]

So it's best to keep the current behaviour, ie. syntax error. I must say the current syntax error is actually pretty accurate too, so it won't be too hard to find the problem either.