Perl5 リファレンス

       select FILEHANDLE

       select  Returns the currently selected filehandle.  Sets
               the current default filehandle for output, if
               FILEHANDLE is supplied.  This has two effects:
               first, a `write' or a `print' without a filehandle
               will default to this FILEHANDLE.  Second,
               references to variables related to output will
               refer to this output channel.  For example, if you
               have to set the top of form format for more than
               one output channel, you might do the following:

                   $^ = 'report1_top';
                   $^ = 'report2_top';

               FILEHANDLE may be an expression whose value gives
               the name of the actual filehandle.  Thus:

                   $oldfh = select(STDERR); $| = 1; select($oldfh);

               Some programmers may prefer to think of
               filehandles as objects with methods, preferring to
               write the last example as:

                   use IO::Handle;

               This calls the select(2) system call with the bit
               masks specified, which can be constructed using
               `fileno' and `vec', along these lines:

                   $rin = $win = $ein = '';
                   vec($rin,fileno(STDIN),1) = 1;
                   vec($win,fileno(STDOUT),1) = 1;
                   $ein = $rin | $win;

               If you want to select on many filehandles you
               might wish to write a subroutine:

                   sub fhbits {
                       my(@fhlist) = split(' ',$_[0]);
                       for (@fhlist) {
                           vec($bits,fileno($_),1) = 1;
                   $rin = fhbits('STDIN TTY SOCK');

               The usual idiom is:

                   ($nfound,$timeleft) =
                     select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

               or to block until something becomes ready just do

                   $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

               Most systems do not bother to return anything
               useful in $timeleft, so calling select() in scalar
               context just returns $nfound.

               Any of the bit masks can also be undef.  The
               timeout, if specified, is in seconds, which may be
               fractional.  Note: not all implementations are
               capable of returning the$timeleft.  If not, they
               always return $timeleft equal to the supplied

               You can effect a sleep of 250 milliseconds this

                   select(undef, undef, undef, 0.25);

               WARNING: One should not attempt to mix buffered
               I/O (like `read' or ) with `select', except as
               permitted by POSIX, and even then only on POSIX
               systems.  You have to use `sysread' instead.