/usr/lib/ruby/vendor_ruby/ is in ruby-sinatra-contrib 1.4.6-2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/sinatra/
/usr/lib/ruby/vendor_ruby/sinatra/contrib/
/usr/lib/ruby/vendor_ruby/sinatra/contrib/setup.rb
/usr/lib/ruby/vendor_ruby/sinatra/contrib/all.rb
/usr/lib/ruby/vendor_ruby/sinatra/contrib/version.rb
/usr/lib/ruby/vendor_ruby/sinatra/namespace.rb
/usr/lib/ruby/vendor_ruby/sinatra/decompile.rb
/usr/lib/ruby/vendor_ruby/sinatra/respond_with.rb
/usr/lib/ruby/vendor_ruby/sinatra/streaming.rb
/usr/lib/ruby/vendor_ruby/sinatra/json.rb
/usr/lib/ruby/vendor_ruby/sinatra/extension.rb
/usr/lib/ruby/vendor_ruby/sinatra/reloader.rb
/usr/lib/ruby/vendor_ruby/sinatra/cookies.rb
/usr/lib/ruby/vendor_ruby/sinatra/config_file.rb
/usr/lib/ruby/vendor_ruby/sinatra/custom_logger.rb
/usr/lib/ruby/vendor_ruby/sinatra/link_header.rb
/usr/lib/ruby/vendor_ruby/sinatra/content_for.rb
/usr/lib/ruby/vendor_ruby/sinatra/engine_tracking.rb
/usr/lib/ruby/vendor_ruby/sinatra/test_helpers.rb
/usr/lib/ruby/vendor_ruby/sinatra/multi_route.rb
/usr/lib/ruby/vendor_ruby/sinatra/capture.rb
/usr/lib/ruby/vendor_ruby/sinatra/contrib.rb