In the interest of not learning...

my $max = $#$ref_a > $#$ref_b ? $#$ref_a : $#$ref_b;
my @blank_a = ("") x @$ref_a;
my @blank_b = ("") x @$ref_b;

my @result = map {
                    [
                       $ref_a->[$_] ? @{$ref_a->[$_]} : @blank_a,
                       $ref_b->[$_] ? @{$ref_b->[$_]} : @blank_b
                    ]
                 }
                 0..$max;
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By the way, anyone have a prettier way to do this?