You should take a look at perllol which contains everything you need to know about multdimensional arrays.

Your last code should be changed to:

for $i (0..$N){
    $results[$i] = [ mult($a,@b[$i]) ];
}
[download]

The [] is called an array reference constructor. Without this constructor your array is called in scalar context and it'll just return the number of elements it contains.

Update Fixed my code, thanks to PrakashK, and remove the push.

- Good luck