evaltest.rb

class Foo
  class_eval <<-EOS, __FILE__, __LINE__
    def bar=(*args, &block)
      puts args.inspect
    end
  EOS
end

Foo.new.bar= 1

gistfile2.txt

 % ruby evaltest.rb
[1]
 % /usr/local/bin/rbx evaltest.rb
[1]