list
----
HAT    KNIFE	1
HAT    KNIFE	7
BALL   KNIFE	3
CAT    CLOTH	1
DOG    CLOTH	2
	
code
----	
my %matrix;
while (<>) {
    my ($id1, $id2, $int) = split;
    $matrix{$id1}{$id2} += $int;
}

foreach $key1 (keys %matrix) {
        foreach $key2 (keys %{%matrix->{$key1}}) {
                print "\$matrix{$key1}{$key2} = $matrix{$key1}{$key2} ";                            
                } 
        print"\n";                                                                                   
        }

output
------
$matrix{CAT}{CLOTH} = 1
$matrix{DOG}{CLOTH} = 2
$matrix{HAT}{KNIFE} = 8
$matrix{BALL}{KNIFE} = 3