set_syntax.rb

class MyHash
  def initialize
  end # initialize
  
  def []=(key, value, opt: nil)
    puts key
    puts value
    puts opt
  end # []=
end # MyHash


h = MyHash.new
h[:hello] = 5,  :opt => true

# ./test.rb:12: syntax error, unexpected =>, expecting end-of-input
# h[:hello] = 5,  :opt => true
#                           ^