if ($card_map[$num]) {
    $card_map[$num]; # return value

 } else {
    $num; # return value

 } # end if
[download]

Here is (one of your) problem(s).

$num will only be returned if you either write return $num, or if it's the last statement in your sub, which it isn't.

Also note that $card_map[$num] will return something for negative values of $num, (in that case it counts the index from the back of the array):

$ perl -wle 'my @card_map = qw(zero one two three); print $card_map[-2
+]'
two
[download]

So you most likely need something like this:

my $return_value = '';
if ($num < 0) {
     $return_value = $negative;
     # continue with the positive value from here on
     $num = -$num;
}

# and append other results here:

$return_value .= $card_map[$num];
return $return_value;
[download]

(I'm also quite sure that you should forget about local for now, and work with my).