output

$ ruby -v test.rb
ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin14]
t.value: accessor-set value. didn't go through the defined method!

test.rb

module TestModule
  define_method "value=".to_sym do |v|
    instance_variable_set("@value".to_sym, "defined method set this value")
  end
end

class Test
  include TestModule
  attr_accessor :value
end

t = Test.new
t.value = "accessor-set value. didn't go through the defined method!"

puts "t.value: #{t.value}"