To expand on Roy Johnson's comment, you are iterating one past the end of your array each time, and autovivifying the new elements. Here's a quick demonstration:

my @array = (
   [ 1, 2, 3 ],
   [ 4, 5, 6 ],
);

print scalar(@array), "\n"; # 2
print $array[2][0],   "\n"; # undef
print scalar(@array), "\n"; # 3
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You can see the new element is automatically added by just accessing a subelement.