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my %pending;

...
  delete $pending{$session_id}{$event};
  delete $pending{$session_id} unless keys %{$pending{$session_id}};
}

sub some_event_handler {
  # set a timer for 10s from now, unless it's already set
...
  # caught the timer; clear the flag
  caught_oneshot_timer($_[STATE]);
}