See the current Perl documentation for lib:IO.
Here is our local, out-dated (pre-5.6) version:

IO - load various IO modules

use IO;

IO provides a simple mechanism to load some of the
IO modules at one go. Currently this includes:
IO::Handle
IO::Seekable
IO::File
IO::Pipe
IO::Socket
For more information on any of these modules, please see its respective documentation.