update: If you really want 2 empty blocks, do something like

{ ; }
{ ; }
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warning: nerdy nitpick ahead :)

I would argue that {;} is not an empty block. I think it's analagous to a set that contains the empty set as it's only element.

but i don't think the compiler makes the distinction.. perl -MO=Terse -e 'if(1){;}' and perl -MO=Terse -e 'if(1){}' given identical output.

/me is confused. Do you mean that ";" is an empty set? Perl doesn't even have sets as part of the language. The ; in my post above is just the end of an empty statement (which returns () or undef, depending on context) to make it clear that it's not supposed to be a hash.
"What should it profit a man, if he should win a flame war, yet lose his cool?"

Using if here is a poor example, since since a block is required by if's syntax. (This ins't C.) Also, compare

{
  print '!';
  redo;
}
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to

if (1) {
  print '!';
  redo;
}
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it would would be useless

That doesn't matter.

despite what people would tell you

That doesn't matter either.

IMHO, it is valid code. Look at it this way:

  # block #1
  {

  }

  # block #2
  {

  }
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See? Two empty blocks waiting for code. It is valid.

Just because no one in their perfect state of mind would use that doesn't make it invalid.

You are mistaken:

/////
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is valid,

{}{}
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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
{

}
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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.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

Well if you feel that strongly about it, submit a patch to make it legal, explaining your reasons and showing it breaks nothing else in perl extensive test harness, then wait for it to be accepted.

...it is better to be approximately right than precisely wrong. - Warren Buffet