From what I can see, the use of curly braces in ambiguous. I.e. it can mean the start of a block, or a hash reference.

The interpreter will try its hardest depending on the context to guess what you mean, but in this instance you have to provide a hint to the interpreter to disambiguate the brace.

sub a
{
  +{ x }
}
[download]

or

sub a
{
   return { x }
}
[download]

should do the trick. I forgot which perl doc I read this in. It think it was perlref, but I'm not sure.