/usr/lib/ is in ruby-rack-cache 1.2-3.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/rack/
/usr/lib/ruby/vendor_ruby/rack/cache/
/usr/lib/ruby/vendor_ruby/rack/cache/response.rb
/usr/lib/ruby/vendor_ruby/rack/cache/request.rb
/usr/lib/ruby/vendor_ruby/rack/cache/cachecontrol.rb
/usr/lib/ruby/vendor_ruby/rack/cache/key.rb
/usr/lib/ruby/vendor_ruby/rack/cache/entitystore.rb
/usr/lib/ruby/vendor_ruby/rack/cache/metastore.rb
/usr/lib/ruby/vendor_ruby/rack/cache/context.rb
/usr/lib/ruby/vendor_ruby/rack/cache/appengine.rb
/usr/lib/ruby/vendor_ruby/rack/cache/storage.rb
/usr/lib/ruby/vendor_ruby/rack/cache/options.rb
/usr/lib/ruby/vendor_ruby/rack/cache.rb
/usr/lib/ruby/vendor_ruby/rack-cache.rb