./pm.pl -- unordered data.txt: CAT_HEADER,SUPPLIER_CODE,CUSTOMER_CODE,F4,F5,F6,F7,F8 CAT_LINE,0001P,ABC12345,20190924,,1,Z,3.36 CAT_LINE,0002P,ABC12345,20190924,,1,Z,3.36 CAT_LINE,0001P,ABC23456,20190924,,1,Z,2.24 CAT_LINE,0002P,ABC23456,20190924,,1,Z,2.24 CAT_LINE,0001P,ABC34567,20190924,,1,Z,2.24 CAT_LINE,0002P,ABC34567,20190924,,1,Z,2.24 -- ordered data: CAT_HEADER,SUPPLIER_CODE,CUSTOMER_CODE,F4,F5,F6,F7,F8 CAT_LINE,0001P,ABC12345,20190924,,1,Z,3.36 CAT_LINE,0001P,ABC23456,20190924,,1,Z,2.24 CAT_LINE,0001P,ABC34567,20190924,,1,Z,2.24 CAT_LINE,0002P,ABC12345,20190924,,1,Z,3.36 CAT_LINE,0002P,ABC23456,20190924,,1,Z,2.24 CAT_LINE,0002P,ABC34567,20190924,,1,Z,2.24