sleep(1) - perldoc - phpman

    sleep EXPR
    sleep   Causes the script to sleep for (integer) EXPR seconds, or
            forever if no argument is given. Returns the integer number of
            seconds actually slept.

            EXPR should be a positive integer. If called with a negative
            integer, "sleep" does not sleep but instead emits a warning,
            sets $! ("errno"), and returns zero.

            "sleep 0" is permitted, but a function call to the underlying
            platform implementation still occurs, with any side effects that
            may have. "sleep 0" is therefore not exactly identical to not
            sleeping at all.

            May be interrupted if the process receives a signal such as
            "SIGALRM".

                eval {
                    local $SIG{ALRM} = sub { die "Alarm!\n" };
                    sleep;
                };
                die $@ unless $@ eq "Alarm!\n";

            You probably cannot mix "alarm" and "sleep" calls, because
            "sleep" is often implemented using "alarm".

            On some older systems, it may sleep up to a full second less
            than what you requested, depending on how it counts seconds.
            Most modern systems always sleep the full amount. They may
            appear to sleep longer than that, however, because your process
            might not be scheduled right away in a busy multitasking system.

            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 "usleep". 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.

            See also the POSIX module's "pause" function.