bar.rb

# lib/tools/bar.rb

module MyApp
  module Tools
    module Bar
      def bar_method(item)
        # ...
        Foo.foo_method  # => Undefined method `foo_method` for MyApp::Tools::Foo.Module
      end
    end
  end
end

foo.rb

# lib/tools/foo.rb

module MyApp
  module Tools
    module Foo
      def foo_method
        # ...
      end
    end
  end
end

helpers.rb

# lib/tools/helpers.rb

module MyApp
  module Tools
    module Helpers
      def my_helper(args)
        args.reject { |item| Bar.bar_method item }
      end
    end
  end
end

tools.rb

# lib/tools/tools.rb

require './lib/tools/foo.rb'
require './lib/tools/bar.rb'
require './lib/tools/helpers.rb'