foo.rb

class Foo
  MY_CONST = "hello".freeze
end

puts Foo::MY_CONST # "hello"
Foo.const_set("MY_CONST", "new const")
puts Foo::MY_CONST # "new const"

class Foo
  def self.my_const
    "hello"
  end
end

puts Foo.my_const # "hello"
Foo.my_const.concat("foobar") # that's what we are afraid of without `.freeze` in constants
puts Foo.my_const # still "hello"