print "ref_a: ", $#$ref_a, "\nref_b: ", $#$ref_b, "\n";
for my $i (0..$#$ref_a) {
    print "Reading element $i\n";
    push @{$ref_a->[$i]}, @{$ref_b->[$i]};
}

##</code><code>##

ref_a: 5
ref_b: 2
Reading element 0
Reading element 1
Reading element 2
Reading element 3
Can't use an undefined value as an ARRAY reference at scriptname.pl line 23