@a=qw(1 0 9 15 6);
@b=qw(    1 1  4 );

##</code><code>##

1 0 9 15 6
     1 1 4
__________+
1 1 1 7 0       (which corresponds to the program logic)

##</code><code>##

1 0 9 15 6
    1 1 4
__________+
10 9 2 7 0     (which corresponds to the rules of math)

##</code><code>##

#!/usr/local/bin/perl
use strict;
use warnings;

my (@a, @b, @result);
my ($a_len, $b_len,$a, $b, $sum);

@a=qw(1 0 9 15 6);          #case1 when @a > @b
@b=qw(    1 1  4 );

#@a = qw(   1  5 9);       #case2 when @a < @b
#@b = qw(10 9  3 6);
                           #case3 when @a == @b
#@a = qw(1 1 6 9 2);
#@b = qw(2 1 4 4 8);

$a_len = @a;               #to make decisions on which case to follow
$b_len = @b;

print "@a\n";
print "@b\n";
print "__________+\n";

while(@a || @b){
        if($a_len > $b_len){
                $a = pop @a;
                $b= @b !=0? pop @b : 0;    #pop $b or assign zero where empty.
                $sum = $a + $b ;
                if($sum >= 10 && @a!=0){
                        $sum -=10;        #assign to sum the ones position and strip the tens position.
                        $a[-1]+=1;        # add one to the following position.
                        }
                unshift @result, $sum;
                        }elsif($a_len < $b_len){
                                $a= @a!=0? pop @a: 0;  # pop $a or assign zero where empty
                                $b = pop @b;
                                $sum = $a + $b ;
                                if($sum >= 10 && @b !=0){
                                        $sum -=10;
                                        $b[-1]+=1;
                                        }
                unshift @result, $sum;
                                }else{
                                        $a = pop @a;
                                        $b = pop @b;
                                        $sum = $a+$b;
                                        if($sum >= 10){
                                                $sum -=10;
                                                $a[-1]+=1;
                                                }
                                        unshift @result, $sum;
                                        }
                }
print "@result";