gistfile1.txt

1.9.3p448 :001 > SOME_CONSTANT_HASH = { abc: 123 }
 => {:abc=>123}
1.9.3p448 :002 > SOME_CONSTANT_HASH[:abc]
 => 123
1.9.3p448 :003 > SOME_CONSTANT_HASH[:abc] = 456
 => 456
1.9.3p448 :004 > SOME_CONSTANT_HASH
 => {:abc=>456}
1.9.3p448 :005 > SOME_FROZEN_CONSTANT_HASH = { abc: 123 }.freeze
 => {:abc=>123}
1.9.3p448 :006 > SOME_FROZEN_CONSTANT_HASH[:abc] = 456
RuntimeError: can't modify frozen Hash
        from (irb):6:in `[]='
        from (irb):6
        from /Users/jay/.rvm/rubies/ruby-1.9.3-p448/bin/irb:16:in `<main>'