Another interpretation would be "when ref_b runs out of data, don't do anything more to ref_a".

If this is what's needed then:

for my $i (0..$#$ref_a) {
    push @{$ref_a->[$i]}, @{$ref_b->[$i] || []};
}
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Ie, if @{$ref_b} hasn't got as many members as @{$ref_a}, then at some point $ref_b->[$i] is undef, so dereference a reference to an empty array in that case.

(I also tried to use || last there (ie exit the loop early) but that comes up with a "bizarre" error message.)

If it's also possible for ref_b to have more members than ref_a, and the extras should be tacked on verbatim, then the range of the for loop needs to be adjusted to cover them. Here's one option which is moderately readable:

for my $i (0.. ($#$ref_a > $#$ref_b ? $#$ref_a : $#$ref_b) ) {
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Handily, extra elements appear in ref_a automatically as needed, so there's no need for a || [] on the first argument to push.

Update: Re-reading the thread made me realise that because of the autovivification in ref_a, ikegami's suggestion does acheive all this (I think) but much more subtly. Too subtle for me the first time round ;-)