P:\test>type t1.csv
1,the,rest
2,the,rest
3,the,rest
4,the,rest
5,the,rest

P:\test>type t2.csv
2,the,rest
4,the,rest
6,the,rest
8,the,rest

##</code><code>##

# 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