Perl: Array in hash

1. Array in hash

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my (%all, $city, $country, $cities);

while (<DATA>) {
    chomp;
    ($city, $country) = split /, /;
    # $all{$country} = [] unless exists $all{$country};
    push @{$all{$country}}, $city;
}

for $country (sort keys %all) {
    print "$country: ";
    $cities = \@{$all{$country}};
    my @order = sort @$cities;
    print $order[0], "\t";
    for my $ID (sort @$cities) {
        print $ID, "\t";
    }
    # print $table{$country};
    print "\n";
}
print Dumper (\%all);


__DATA__
Chicago, USA
Frankfurt, Germany
Berlin, Germany
Washington, USA
Helsinki, Finland
New York, USA

Output:

Finland: Helsinki Helsinki  
Germany: Berlin Berlin  Frankfurt 
USA: Chicago  Chicago New York  Washington  
$VAR1 = {
          'Finland' => [
                         'Helsinki'
                       ],
          'Germany' => [
                         'Frankfurt',
                         'Berlin'
                       ],
          'USA' => [
                     'Chicago',
                     'Washington',
                     'New York'
                   ]
        };