You are mistaken:

/////
[download]

is valid,

{}{}
[download]

is not :-)

The fact that it looks valid doesn't make it valid. The perl interpreter gets to decide. There is no ANSI perl :-) Now, I agree that your construct could be made valid, but there are other situations to consider:

# hash #1
{

}
[download]

What about this? Is this a block or a hash? Current semantics say it's a hashref, your interpretation says its a block. Or do you really want this to be an empty block only when it's followed by another block? Would that help catch typos or would it confuse people even more than the current semantics? I must say I'm not sure.