class Foo

  BAR = {a: []}

  def initialize
    @whatever = BAR.dup
    #...
    @whatever[:a].push "foo"
    puts @whatever[:a].length
  end

end

Foo.new #output: 1
Foo.new #output: 2
Foo.new #output: 3
Foo.new #output: 4
Foo.new #output: 5