use strict;
    use warnings;

    # Hash of countries by city
    my %countries_by = (
        'Chicago'    => 'USA',
        'Frankfurt'  => 'Germany',
        'Berlin'     => 'Germany',
        'Washington' => 'USA',
        'Helsinki'   => 'Finland',
        'New York'   => 'USA',
    );

    # Hash of arrays of cities by country
    my %cities_by;

    # Generate a hash of arrays of cities by country
    # from a hash of countries by city...
    while (my ($city, $country) = each %countries_by) {
        push @{ $cities_by{$country} }, $city;
    }

    # Print a list of cities by country...
    for my $country (sort keys %cities_by) {
        for my $city (sort @{ $cities_by{$country} }) {
            print "$country\t$city\n";
        }
    }