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";
}