It is not adding the values related to one key. Is is possible to make the key unique and let all the values for that key be associated to the key and then sum all the values related to that key

In the data you provided there is no duplicate keys, so no adding will happen. The key is the concatenation of $subc and $state and this combination creates unique keys for your data.
See below where I integrated you MAP and REDUCE into on loop.

use strict;
use warnings;

my %worth;
while ( <DATA> ) {
        chomp;
        my( $txnid, $date, $custid, $amount, $productc, $subc, $city, 
+$state, $mode ) = split /,/;
        $worth{"$subc.$state"} += $amount; # REDUCE
}
while( my ( $key, $value) = each %worth ) {
        print "key: $key, value: $value\n";
}

__DATA__
00000000,06-26-2011,4007024,040.33,Exercise & Fitness,Cardio Machine A
+ccessories,Clarksville,Tennessee,credit
00000001,05-26-2011,4006742,198.44,Exercise & Fitness,Weightlifting Gl
+oves,Long Beach,California,credit
00000002,06-01-2011,4009775,005.58,Exercise & Fitness,Weightlifting Ma
+chine Accessories,Anaheim,California,credit
00000003,06-05-2011,4002199,198.19,Gymnastics,Gymnastics Rings,Milwauk
+ee,Wisconsin,credit
00000004,12-17-2011,4002613,098.81,Team Sports,Field Hockey,Nashville 
+ ,Tennessee,credit
00000005,02-14-2011,4007591,193.63,Outdoor Recreation,Camping & Backpa
+cking & Hiking,Chicago,Illinois,credit
00000006,10-28-2011,4002190,027.89,Puzzles,Jigsaw Puzzles,Charleston,S
+outh Carolina,credit
00000007,07-14-2011,4002964,096.01,Outdoor Play Equipment,Sandboxes,Co
+lumbus,Ohio,credit
00000008,01-17-2011,4007361,010.44,Winter Sports,Snowmobiling,Des Moin
+es,Iowa,credit
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