G'day ahjohnson2,

The following achieves the results specifed in your OP and in your subsequent update.

Note the different data structure I've used. The order of continents is important so I've used an array of hashrefs each with a single continent key. The order of cities is also important so the values of those keys are arrayrefs.

[Also note that you've misspelled "Cairo" two different ways — this will cause errors that may be hard to track down — taking care over such details is important.]

#!/usr/bin/env perl -l

use strict;
use warnings;

use Data::Dump;

my @parent = (
    { Europe => [ qw{ London Moscow } ] },
    { Asia => [ qw{ Tokyo Seoul } ] },
    { Africa => [ qw{ Cairo Algiers } ] },
);

my @current;
reset_current(\@parent, \@current);

print '*** Starting State ***';
print_current_state(\@current);

print '*** Asia Down ***';
failover(Asia => \@current);
print_current_state(\@current);

print '*** Reset State ***';
reset_current(\@parent, \@current);
print_current_state(\@current);

print '*** Europe Down ***';
failover(Europe => \@current);
print_current_state(\@current);

print '*** Asia Down ***';
failover(Asia => \@current);
print_current_state(\@current);

sub failover {
    my ($down, $current) = @_;

    my @cities_to_failover
        = map { (keys %$_)[0] eq $down ? @{$_->{$down}} : () } @$curre
+nt;

    for (@$current) {
        my ($continent) = keys %$_;
        next unless @{$_->{$continent}};

        if ($continent eq $down) {
            @{$_->{$continent}} = ();
        }
        else {
            push @{$_->{$continent}}, shift @cities_to_failover;
        }
    }
}

sub print_current_state {
    my ($current) = @_;
    dd $current;
}

sub reset_current {
    my ($parent, $current) = @_;

    @$current = ();

    for (@$parent) {
        my ($continent) = keys %$_;
        push @$current, { $continent => [ @{$_->{$continent}} ] };
    }
}
[download]

Output:

*** Starting State ***
[
  { Europe => ["London", "Moscow"] },
  { Asia => ["Tokyo", "Seoul"] },
  { Africa => ["Cairo", "Algiers"] },
]

*** Asia Down ***
[
  { Europe => ["London", "Moscow", "Tokyo"] },
  { Asia => [] },
  { Africa => ["Cairo", "Algiers", "Seoul"] },
]

*** Reset State ***
[
  { Europe => ["London", "Moscow"] },
  { Asia => ["Tokyo", "Seoul"] },
  { Africa => ["Cairo", "Algiers"] },
]

*** Europe Down ***
[
  { Europe => [] },
  { Asia => ["Tokyo", "Seoul", "London"] },
  { Africa => ["Cairo", "Algiers", "Moscow"] },
]

*** Asia Down ***
[
  { Europe => [] },
  { Asia => [] },
  { Africa => ["Cairo", "Algiers", "Moscow", "Tokyo"] },
]
[download]

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