/usr/lib/ruby/ is in librmail-ruby1.8 0.17-1.1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/1.8/
/usr/lib/ruby/1.8/rmail.rb
/usr/lib/ruby/1.8/rmail/
/usr/lib/ruby/1.8/rmail/parser.rb
/usr/lib/ruby/1.8/rmail/message.rb
/usr/lib/ruby/1.8/rmail/header.rb
/usr/lib/ruby/1.8/rmail/address.rb
/usr/lib/ruby/1.8/rmail/mailbox.rb
/usr/lib/ruby/1.8/rmail/serialize.rb
/usr/lib/ruby/1.8/rmail/utils.rb
/usr/lib/ruby/1.8/rmail/mailbox/
/usr/lib/ruby/1.8/rmail/mailbox/mboxreader.rb
/usr/lib/ruby/1.8/rmail/parser/
/usr/lib/ruby/1.8/rmail/parser/multipart.rb
/usr/lib/ruby/1.8/rmail/parser/pushbackreader.rb