# SYNOPSYS
#   
#   # start new alarm (fork new process)
#   $alarm_id = alarm_start(0.01, \$counter);
#   
#   # stop alarm (kill forked process)
#   alarm_stop($alarm_id);
#   
#   # sleep without been interrupted by signals
#   sleepy(0.5);
#   
#   # limit CPU usage to 10%
#   Limit_CPU();
#   
use Time::HiRes qw(time alarm setitimer ITIMER_PROF);
{ my %ALARM;
sub alarm_start {
    my ($PARENT,   $time, $counter) = ($$,   @_);
    return unless $time > 0;
    return unless my ($n) = 
        grep { not exists $ALARM{$_} } "NUM50" .. "NUM59";
    $SIG{$n} =  ref($counter) eq "SCALAR"   ? sub { $$counter++ }   :
                ref($counter) eq "CODE"     ?
                    sub { local $SIG{$n}="IGNORE"; &$counter }      :
                return;
    return $n if $ALARM{$n} = fork();
    $0 .= " ALARM $n";
    $SIG{ALRM} = sub { kill $n, $PARENT or exit };
    alarm($time, $time);
    CORE::sleep 1 while 1;
}
sub alarm_stop {
    return unless exists $ALARM{$_[0]};
    local $SIG{CHLD} = $SIG{$_[0]} = "IGNORE";
    kill 9, $ALARM{$_[0]} and waitpid delete $ALARM{$_[0]}, 0;
}}
sub sleepy {
    my $alarm = alarm_start($_[0], \(my $wakeup)) or 
        die "sleepy: alarm failed"; # die or return?
    CORE::sleep 1 until $wakeup;
    alarm_stop($alarm);
}
use constant CPUMAX => 10;  # percent, max CPU load by this process
use constant INT    => 0.1; # sec, how often check for CPU overload
{ my ($real, $prof, $wait);
$SIG{PROF} = sub { $prof++ };
sub Limit_CPU {
    setitimer(ITIMER_PROF, INT, INT);
    alarm_start(INT, sub {
        $wait = ($prof*(100/CPUMAX) - ++$real) / (1/INT);
        return if INT > $wait;
        sleepy($wait);
        $real += int( $wait * (1/INT) );
        });
}}