...
       Event-driven I/O works best for filehandles that have been
       placed into nonblocking mode.  In blocking mode, a "print"
       command may block if you give it more data than the under­
       lying file or device can accept, and a "<>", "sysread" or
       "read" command will block if you attempt to read more data
       than is ready;  no events will be processed while the com­
       mands block.  In nonblocking mode "print", "<>", "sysread"
       and "read" never block.  See the documentation for the
       individual commands for information on how they handle
       blocking and nonblocking filehandles.