phpMan > perldoc > alarm(0)

    alarm SECONDS
    alarm   Arranges to have a SIGALRM delivered to this process after the
            specified number of wallclock seconds has elapsed. If SECONDS is
            not specified, the value stored in $_ is used. (On some
            machines, unfortunately, the elapsed time may be up to one
            second less or more than you specified because of how seconds
            are counted, and process scheduling may delay the delivery of
            the signal even further.)

            Only one timer may be counting at once. Each call disables the
            previous timer, and an argument of 0 may be supplied to cancel
            the previous timer without starting a new one. The returned
            value is the amount of time remaining on the previous timer.

            For delays of finer granularity than one second, the Time::HiRes
            module (from CPAN, and starting from Perl 5.8 part of the
            standard distribution) provides "ualarm". You may also use
            Perl's four-argument version of "select" leaving the first three
            arguments undefined, or you might be able to use the "syscall"
            interface to access setitimer(2) if your system supports it. See
            perlfaq8 for details.

            It is usually a mistake to intermix "alarm" and "sleep" calls,
            because "sleep" may be internally implemented on your system
            with "alarm".

            If you want to use "alarm" to time out a system call you need to
            use an "eval"/"die" pair. You can't rely on the alarm causing
            the system call to fail with $! set to "EINTR" because Perl sets
            up signal handlers to restart system calls on some systems.
            Using "eval"/"die" always works, modulo the caveats given in
            "Signals" in perlipc.

                eval {
                    local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
                    alarm $timeout;
                    my $nread = sysread $socket, $buffer, $size;
                    alarm 0;
                };
                if ($@) {
                    die unless $@ eq "alarm\n";   # propagate unexpected errors
                    # timed out
                }
                else {
                    # didn't
                }

            For more information see perlipc.

            Portability issues: "alarm" in perlport.