my @increments = (undef, 30, 300, 30000);
my @times = map {($_, time + $increments[$_])} 1..3;
alarm($times[0][1]);
#...
# then in sigalarm, handle and reschedule the next phase
my $this_phase = (shift(@times))->[0];
@times = sort { $a->[1] <=> $b[1] } (@times, [$this_phase, $increments[$this_phase]);
alarm($times[0][1]);
my_garbage_collect($this_phase);