Two quick suggestions. First, instead of sorting the list of keys and then reversing it, sort in the proper order to begin with. Second, foreach (LIST) { push ARRAY, $_ } can be rewritten as push ARRAY LIST.

foreach my $host (sort { $frequency{$b} <=> $frequency{$a} } keys %fre
+quency) {
  push @recipients, @$host;
}
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