my @timeSlots;
my $concurrent = 0;
my $lastTimeSlot = 0;

while( <$log> ) {
    my( $time, $type ) = m[(...).+(Start|End)];
    ++$concurrent if $type eq 'Start';
    --$concurrent if $type eq 'End';
    my $timeSlot = timeToTimeslot( $time );
    push @timeSlots, ( $concurrent ) x ( $timeSlot - $lastTimeSlot );
    $lastTimeSlot = $timeSlot;

    ## Do stuff to build your intspan objects
    ...
}