Using Laurent_R's hash solution, combined with proper CSV parsing, I'd propose:

use Text::CSV_XS qw( csv );

my %acc;
my $aoa = csv (in => "test.csv", on_in => sub {
    $acc{pack "L>L>", $_[1][0], $_[1][1]} += $_[1][2]; });
csv (in => [ map { [(unpack "L>L>", $_), $acc{$_}] } sort keys %acc ])
+;
[download]