irb(main):001:0> $LOADED_FEATURES << 'abc.rb'
=> ["enumerator.so", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/enc/encdb.so", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/enc/trans/transdb.so", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/enc/iso_8859_1.so", "E:/installs/ruby191p376/lib/ruby/1.9.1/rubygems.rb", "E:/installs/ruby191p376/lib/ruby/gems/1.9.1/gems/faster_rubygems-0.1.0/lib/faster_rubygems.rb", "E:/installs/ruby191p376/lib/ruby/gems/1.9.1/gems/faster_rubygems-0.1.0/lib/ubygemsf.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/rbconfig.rb", "E:/installs/ruby191p376/lib/ruby/gems/1.9.1/gems/backtracer-0.7.2/lib/shared.rb", "E:/installs/ruby191p376/lib/ruby/gems/1.9.1/gems/backtracer-0.7.2/lib/backtracer.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/e2mmap.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/init.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/workspace.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/context.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/extend-command.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/output-method.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/notifier.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/slex.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/ruby-token.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/ruby-lex.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/src_encoding.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/magic-file.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/enc/euc_jp.so", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/enc/shift_jis.so", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/etc.so", "E:/installs/ruby191p376/lib/ruby/1.9.1/i386-mingw32/dl.so", "E:/installs/ruby191p376/lib/ruby/site_ruby/1.9.1/rbreadline.rb", "E:/installs/ruby191p376/lib/ruby/site_ruby/1.9.1/readline.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/input-method.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb/locale.rb", "E:/installs/ruby191p376/lib/ruby/1.9.1/irb.rb", "abc.rb"]
irb(main):002:0> require 'abc'
=> false

this could be optimized more for 1.9

Also, could loaded_features be made into a hash for much faster comparison?