def instanciator(class_or_string)
  if class_or_string.instance_of?(String)
    return Kernel.get_const(class_or_string).new
  else
    return class_or_string.new
end

klass = "MyClass"
o = instanciator(klass)

# same as

klass = MyClass
o = instanciator(klass)