Net::RBLClient's listed_by() method returns a list of servers blocking an IP. You assign this to an array, and assign a reference to that array to an element of your hash. If you want to iterate over that array, you can just use something like the following instead of your foreach:

#This line isn't needed anymore
#my $blackholes = $tpl->new_loop("listings", $omr);

for my $server ( @{$rbl_list{$omr}} )
{
    #do something for each server in the array
}
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