block_args_for_square_brackets.rb

class Thing

  def [] a, &block
    block.call a
  end

end

thing = Thing.new

# explicit works fine
thing[:a,&(-> a { puts a })]

# implicit won't even parse
# thing[:a] { |a| puts a }

There is nothing wrong with the syntax, it is just that Ruby19 does not support this correctly yet (ref http://nuclearsquid.com/writings/ruby-1-9-what-s-new-what-s-changed/)

I'd rewrite it to:

def [] a
  yield a if block_given?
end