select FILEHANDLE
    select  Returns the currently selected filehandle. If FILEHANDLE is
            supplied, sets the new current default filehandle for output.
            This has two effects: first, a "write" or a "print" without a
            filehandle default to this FILEHANDLE. Second, references to
            variables related to output will refer to this output channel.

            For example, to set the top-of-form format for more than one
            output channel, you might do the following:

                select(REPORT1);
                $^ = 'report1_top';
                select(REPORT2);
                $^ = '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;
                STDERR->autoflush(1);

            Portability issues: "select" in perlport.

    select RBITS,WBITS,EBITS,TIMEOUT
            This calls the select(2) syscall 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 may wish to write
            a subroutine like this:

                sub fhbits {
                    my @fhlist = @_;
                    my $bits = "";
                    for my $fh (@fhlist) {
                        vec($bits, fileno($fh), 1) = 1;
                    }
                    return $bits;
                }
                $rin = fhbits(*STDIN, *TTY, *MYSOCK);

            The usual idiom is:

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

            or to block until something becomes ready just do this

                $nfound = select($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 $timeout.

            You can effect a sleep of 250 milliseconds this way:

                select(undef, undef, undef, 0.25);

            Note that whether "select" gets restarted after signals (say,
            SIGALRM) is implementation-dependent. See also perlport for
            notes on the portability of "select".

            On error, "select" behaves just like select(2): it returns -1
            and sets $!.

            On some Unixes, select(2) may report a socket file descriptor as
            "ready for reading" even when no data is available, and thus any
            subsequent "read" would block. This can be avoided if you always
            use O_NONBLOCK on the socket. See select(2) and fcntl(2) for
            further details.

            The standard "IO::Select" module provides a user-friendlier
            interface to "select", mostly because it does all the bit-mask
            work for you.

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

            Portability issues: "select" in perlport.