In your main loop, you check that the time difference is exactly 1800, 3600, etc. seconds. This is a condition that is only true for one second. And since you are sleeping for a full second, it seems conceivable that if the system is very busy, your process may not get woken up from its sleep in that one-second window of opportunity.

I might rewrite the loop in the following way to be sure that the action is triggered the first time the script is woken up after a target number of seconds.

sleep 1 until time - $llent->ll_time >= 1800;
passive_pop("Thirty minutes");

sleep 1 until time - $llent->ll_time >= 3600;
passive_pop("One hour");

sleep 1 until time - $llent->ll_time >= 5400;
passive_pop("Ninety minutes");

sleep 1 until time - $llent->ll_time >= 7200;
passive_pop("Two hours");
tattle_tail();
[download]

Or to be even fancier and extensible-er:

my %actions = (
  1800 => sub { passive_pop("Thirty minutes"); },
  3600 => sub { passive_pop("Sixty minutes"); },
  5400 => sub { passive_pop("Ninety minutes"); },
  7200 => sub { passive_pop("Two hours"); tattle_tail(); },
# 8000 => sub { disable_network_interface(); },
# 9999 => sub { self_destruct(); }
);

for my $duration (sort {$a <=> $b} keys %actions) {
  sleep 1 until time - $llent->ll_time >= $duration;
  $actions{$duration}->();
}
[download]

Also, it appears that your code as written will loop forever. This may be fine if your KDE autostart things get automatically killed when he logs out, but is something to consider.