my @captured;
While I still have records:
  Is this a break line?
    Y - check_for_more_breaks(@captured)
        @captured = ()
        next item
    N - save line to @captured
Anything left in @captured?
   check_for_more_breaks(@captured)

sub check_for_more_breaks:
  # Your actual processing here is unclear; I've taken
  # a guess at it.
  my @subgroup;
  my $last_sequence_number;
  for each line:
     extract the fields
     is last_sequence number defined?
         N - save this one as the last sequence number
             save the current data of interest in @subgroup
             next
     else is there a gap in the sequence numbers?
         Y - summarize(@subgroup)
             @subgroup = ();
             save current sequence number as last
             save the current data of interest
             next