test.rb

irb(main):001:0> module MyMod; define_method(:foo) do |test='test', bah| puts test; puts bah; end; end
=> #<Proc:0x00000001bbb470@(irb):1 (lambda)>
irb(main):002:0> include MyMod
=> Object
irb(main):003:0> foo
ArgumentError: wrong number of arguments (0 for 1)
  from (irb):1:in `block in <module:MyMod>'
	from (irb):3
	from /usr/local/bin/irb:12:in `<main>'
irb(main):004:0> foo('asdf')
test
asdf
=> nil