Take advantage of metaprogramming and avoid littering the logic of your code with `define_method` declarations.

attr_capcessor.rb

module Cappy
  def attr_capcessor attr
    attr_reader attr.to_sym

    define_method "#{attr}=" do |value|
      instance_variable_set("@#{attr}", value.to_s.upcase)
    end
  end
end

class A
  extend Cappy
  attr_capcessor :body
end

a = A.new
lower_case_val = "wants to be uppcased"
a.body = lower_case_val

STDERR.puts "SAVED:     #{lower_case_val}"
STDERR.puts "Retrieved: #{a.body}"

# OUTPUT
#
# $ ruby attr_capcessor.rb
#
# SAVED:     wants to be uppcased
# Retrieved: WANTS TO BE UPPCASED