C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV>type hello.csv
abc,2,3,4,5,6
abc,7,5,2,1,6,2,3
abc,8,2,1,3,1,4
def,8,9,4,5,6
def,5,6,2,1

C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV>perl mergetest99.pl hello.csv

C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV>type hello.csv.out.csv
abc,2,3,4,5,6,7,5,2,1,6,2,3,8,2,1,3,1,4
def,8,9,4,5,6,5,6,2,1