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Sends a SIGALRM signal to the executing Perl program after n seconds. On some older systems, alarms go off "at the top of the second," so, for instance, an alarm 1 may go off anywhere between 0 to 1 seconds from now, depending on when in the current second it is. An alarm 2 may go off anywhere from 1 to 2 seconds from now. And so on.

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 return value is the number of seconds remaining on the previous timer.