#! perl
use strict;
use warnings;

my %h1 = 
(
    'Chicago, USA'       => undef,
    'Frankfurt, Germany' => undef,
    'Berlin, Germany'    => undef,
    'Washington, USA'    => undef,
    'Helsinki, Finland'  => undef,
    'New York, USA'      => undef,
);

my %h2;

for (keys %h1)
{
    my  ($city, $country) = split ', ';
    push @{ $h2{$country} }, $city;
}

for (sort keys %h2)
{
    print "$_: ", join(', ',  sort { $a cmp $b } @{ $h2{$_} }), "\n";
}