/usr/lib/ is in ruby-ref 1.0.2+dfsg-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/ref/
/usr/lib/ruby/vendor_ruby/ref/weak_reference.rb
/usr/lib/ruby/vendor_ruby/ref/abstract_reference_value_map.rb
/usr/lib/ruby/vendor_ruby/ref/reference.rb
/usr/lib/ruby/vendor_ruby/ref/abstract_reference_key_map.rb
/usr/lib/ruby/vendor_ruby/ref/weak_reference/
/usr/lib/ruby/vendor_ruby/ref/weak_reference/pure_ruby.rb
/usr/lib/ruby/vendor_ruby/ref/weak_reference/iron_ruby.rb
/usr/lib/ruby/vendor_ruby/ref/weak_reference/weak_ref.rb
/usr/lib/ruby/vendor_ruby/ref/soft_key_map.rb
/usr/lib/ruby/vendor_ruby/ref/weak_key_map.rb
/usr/lib/ruby/vendor_ruby/ref/soft_value_map.rb
/usr/lib/ruby/vendor_ruby/ref/mock.rb
/usr/lib/ruby/vendor_ruby/ref/soft_reference.rb
/usr/lib/ruby/vendor_ruby/ref/weak_value_map.rb
/usr/lib/ruby/vendor_ruby/ref/safe_monitor.rb
/usr/lib/ruby/vendor_ruby/ref/reference_queue.rb
/usr/lib/ruby/vendor_ruby/ref/strong_reference.rb
/usr/lib/ruby/vendor_ruby/ref.rb