The issue is that you are trying to access an array element like a hash element. You put array refs into a hash here:

$rbl_list{$ip} = \@listed_by;

And later access like this:

@{$rbl_list{$omr}{blackholes}

$rbl_list{$omr} is an array ref, so $rbl_list{$omr}{blackholes} is like saying $arrayref->{$somekey}. (You also probably want $blackholes not the bareword blackholes)

Edit: I am undone by corion's speed.