my %list;
$list[1]=[1,2,6];
$list[2]=[4,5];
$list[3]=[7,3];

my $n = 1;
foreach my $A (@{$list[1]}) {
    foreach my $B (@{$list[2]}) {
        foreach my $C (@{$list[3]}) {
            print "comb[$n] = [$A,$B,$C]\n";
            $n++;
        }
    }
}