-- raw data table for testing.  I added a column called autoId which you indicated you have
-- in your table.
create table #rawdata (data varchar(8000), autoId int not null identity(1,1));

insert into #rawdata values ('H0001BOBJOHNSON');
insert into #rawdata values ('DXXXABananas');
insert into #rawdata values ('DXXXBApples');
insert into #rawdata values ('DXXXCPears');
insert into #rawdata values ('H0002SUEJONES');
insert into #rawdata values ('H0003STEVESMITH');
insert into #rawdata values ('DXXXAMelons');
insert into #rawdata values ('DXXXBOranges');

-- see what is in the raw data table.
select * from #rawdata;

-- header data table.  The headerId is the same id in the raw data table. 
-- detailedRowcount is the number of detail rows for this customer.
-- headerAutoId is an autoId for this table.
create table #header (custId varchar(32), custName varchar(255), 
   headerId int, detaliedRowCount int, headerAutoId int not null identity(1,1));
   
-- Parse and insert the header rows.
insert into #header
select
   substring(data,2,4),
   stuff(data,1,5,''),
   autoId,
   null
from
   #rawdata
where
   data like 'H%';

-- autoId of the last record in the raw data table. I need this to 
-- figure out how many details rows are there for the last header row.
declare @rawRecordCount int;
select @rawRecordCount = max(autoId) from #rawdata;

-- update with number of detail rows that belong to each header row.
update a set
   detaliedRowCount = coalesce(b.headerId,@rawRecordCount) - a.headerId
from
   #header a
   left join #header b on a.headerAutoId + 1 = b.headerAutoId;
   
select * from #header;

-- the detail table.
create table #detail (trxnId varchar(32), trxnName varchar(255), custId varchar(32));

-- parse and insert the detail rows into detail table
insert into #detail
select
   substring(a.data,2,4),
   stuff(a.data,1,5,''),
   b.custId
from
   #rawdata a
   inner join #header b on a.autoId > b.headerId and a.autoId <= b.headerId + b.detaliedRowCount
where
   a.data like 'D%'
   
select * from #detail;

-- and that should be it.
drop table #rawdata;
drop table #detail;
drop table #header;

perl -pi.bak -e "s/\n/,$.\n/g" YourFile.txt