First, it seems like this is a situation for a hash of arrays (see perllol and HASHES OF ARRAYS). So your data might look like: 
my %parent = ( 
    europe => [qw|
                london
                moscow
               |],
    asia   => [qw|
                tokyo
                seoul
               |],
    africa => [qw|
                cario
                algiers
               |],
     );

[download]
though a hash of hashes might make sense depending on how you access the data.

Second, do you care about determinism/repeatability in where a city rolls over to? You could just cycle over keys, and push the cities onto the next taker: 

sub fallover {
    my ($hash, $failure) = @_;
    my $list = delete($hash->{$failure}) or return;
    my @targets = keys %$hash;
    my $i = 0;
    while (@$list) {
        my $city = shift @$list;
        push @{$hash->{$targets[$i++]}}, $city;
        $i %= @targets;
    }
}

[download]

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re: Scripting Failover Scenario by kennethk
in thread Scripting Failover Scenario by ahjohnson2

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