foo.rb

class Hash
  def self.setter(*args)
    -> (paths, val, obj) {
      obj.deep_dup.tap do |o|
        *dir, final = paths
        if dir.empty?
          o.store(final, val)
        else
          o.dig(*dir).store(final, val)
        end
      end
    }.curry[*args]
  end
end

a = { foo: 23 }
Hash.setter [:foo], 42, a
p a #=> {:foo=>42}