Your ip_sort function is not well suited for what you want to do, as it is designed to be called directly from sort without any intermediate layer (i.e. sort ip_sort, @ips). But you can modify it easily to also support your case (look for $$ inside the documentation for the sort builtin to understand how it works):

sub ip_cmp ($$) {
  my @a = split /\./, $_[0];
  my @b = split /\./, $_[1];

  return $a[0] <=> $b[0]
      || $a[1] <=> $b[1]
      || $a[2] <=> $b[2]
      || $a[3] <=> $b[3];
}
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And then, you will also be able to use it as follows:

my @sorted_keys = sort { ip_cmp($hash{$a}->ip_address, $hash{$b}->ip_a
+ddress) } keys %hash;
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Finally, you can also use the module Sort::Key::IPv4, specially if the hash is big and sorting speed matters:

use Sort::Key::IPv4 qw(ipv4keysort);
my @sorted_keys = ipv4keysort { $hash{$_}->ip_address } keys %hash;
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