The long way is to use a bunch of for loops:

for my $acct_id (keys %customers) {
  print "Account: $acct_id\n";
  for my $ip (keys %{$customers{$acct_id}}) {
    # Do stuff here with IP
    print "IP: $ip\n";
    for my $id (keys %{$customers{$acct_id}{$ip}}) {
      print "ID: $id\n";
    }
  }
}
[download]

As I frequently seem to be specifying, I like to visualize my data structure and how it's layed out. Just to ensure you are properly visualing your data structure, you may want to take a look at Re: How can I visualize my complex data structure?. Then take advantage of Data::Dumper to ensure that you are traversing your data structures in the proper manner.

Eric