Comment by shagie

You can freeze strings that are created at runtime.

    irb(main):001> str = "f".concat("o").concat("o")
    => "foo"
    irb(main):002> str.frozen?
    => false
    irb(main):003> str.freeze
    => "foo"
    irb(main):004> str.frozen?
    => true
    irb(main):005> str = str.concat("bar")
    (irb):5:in 'String#concat': can't modify frozen #<Class:#<String:0x000000015807ec58>>: "foo" (FrozenError)
     from (irb):5:in '<main>'
     from <internal:kernel>:168:in 'Kernel#loop'
     from /opt/homebrew/Cellar/ruby/3.4.4/lib/ruby/gems/3.4.0/gems/irb-1.14.3/exe/irb:9:in '<top (required)>'
     from /opt/homebrew/opt/ruby/bin/irb:25:in 'Kernel#load'
     from /opt/homebrew/opt/ruby/bin/irb:25:in '<main>'