while ( <MYFILE> ) {
   chomp;
   (my $txnid,my $date,my $custid,my $amount, my $productc, my $subc, my $city, my $state, my $mode) = split(",");
   my $key_to_reduce = join ".", "$subc", "$state";
   print "$key_to_reduce\t$amount\n";
}

##</code><code>##

while ( <MYFILE> ) {
   # not needed if you don't use last field 
   # chomp;

   # one 'my' for a list; grab only the needed fields
   my ($amount, $subc, $state) = (split/,/)[3,5,7];

   # no need for join:
   my $key_to_reduce           = "$subc.$state";

   print "$key_to_reduce\t$amount\n";
}