test.rb

module Blah
  def configuration
    @configuration ||={}
  end

  def configure(&block)
    yield configuration
  end

  def register_handler(name, &block)
    config = configuration.dup
    yield config
  end
end

module Other
  extend Blah

  configure do |c|
    c[:username] = 'this'
  end

  register_handler(:other) do |c|
    c[:username] = 'blah'
  end

  p configuration
end