sub next_workday_9am {
   my ($dt) = @_;

   $dt = $dt->add(days => 1)
      if $dt->hour() >= 9;

   my $wday = $dt->wday();
   if    ($wday == 6) { $dt = $dt->add(days => 2); }
   elsif ($wday == 7) { $dt = $dt->add(days => 1); }

   $dt = $dt->truncate(to => 'day')->set(hour => 9);

   return $dt;
}


my $sla_period = DateTime::SpanSet->from_set_and_duration(
      set => DateTime::Set->from_recurrence(
         recurrence => \&next_workday_9am,
      ),
      hours => 8,
   );
);


my $outage_start = 1124424000;
my $outage_end   = 1140807162;

my $outage = DateTime::Span->from_datetimes(
   start  => DateTime->from_epoch($outage_start), 
   before => DateTime->from_epoch($outage_end),
);


print($sla_period->intersection($outage)->duration(), "\n");
[download]

DateTime::Set and related modules are definitely the way to go, although they are very daunting at first.