sleep
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.
For delays of finer granularity than one second, you may use Perl's
syscall interface to access setitimer if your
system supports it, or else see select() below.
See also the
POSIX module's sigpause function.