my %parent = ( 
    europe => [qw|
                london
                moscow
               |],
    asia   => [qw|
                tokyo
                seoul
               |],
    africa => [qw|
                cario
                algiers
               |],
     );

##</code><code>##

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;
    }
}