# 23456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 1234
perl -anF, -le"$h{$F[0]}=1;eof&&last}{$h{(split',',$_)[0]}--while<>;$h{$_}&&print $h{$_}<0?'1:':'2:',$_ for keys%h" t1.csv t2.csv
1:8
1:6
2:1
2:3
2:5