a.rb

class A
  def foo
    case true
    when true
      puts defined?(bar)
      puts "nil? #{bar}"
    when false
      bar = "something else"
      raise "here? no"
    end

    puts defined?(bar)
    puts "nil? #{bar.inspect}"
  end

  def bar
    @bar ||= "baz"
  end
end

A.new.foo

output.txt

$ ruby tmp/a.rb 
method
nil? baz
local-variable
nil? nil