You can require a file with class description in a method, it works. But you can't include a class description directly in a method, it will cause SyntaxError, please check https://gist.github.com/4431884

gistfile1.rb

// main file
def f
  require '2'
  puts C.new.g
end

f # => 'hello world'

// 2.rb
class C
  def g
    'hello world'
  end
end