This is a lovely overview. Hash is a great example of how delightful it can be to program in Ruby.

One more technique worth noting is the chained functional style of using Hash, which you can do in Ruby because Hash inherits from Enumerable. If you're prototyping a script to do some data-cleaning, this makes it easy to build up your pipeline and iterate on it. For example:

    foobar = { ...your data here... }
    foobar.map do |k, v|
      # ...
      # do some transformation here
      # ...
      # and then return the replacement key/value for this entry
      [key, new_value]
    end.select do |k, v|
      # do some filtering here, e.g.:
      is_foobarable?(k, v)
    end.map do |k, v|
      # ...
      # do some expensive transformation here on the smaller data set
      # ...
      [key, newer_value]
    end.to_h

(Note that you have to call #to_h at the end since the Enumerable functions will coerce the hash into an array of arrays.)

Now your code literally shows the pipeline that your data is falling through — and each of these steps can be side-effect-free, with no mutations to the original foobar structure.