It looks like you are putting a SQL expression as a string into the LastUpdate property. That won't work - it needs to be a string that is convertible to a datetime. Try this:
my $time = '2014-04-17T17:31:18';
my $result = $swql->swis->Update($alert->{uri}, {
'TriggerCount'=> $alert->{triggercount} + 1,
'LastUpdate'=> $time,
'Notes' => $notes,
});