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    (
        'Chicago'    => 'USA',
...
        'Helsinki'   => 'Finland',
        'New York'   => 'USA',
    )

    (
        'Finland' => [ 'Helsinki' ],
        'Germany' => [ 'Berlin', 'Frankfurt' ],
        'USA'     => [ 'Chicago', 'New York', 'Washington' ],
    )

    use strict;
    use warnings;
...
            print "$country\t$city\n";
        }
    }