my $var = [
 {
  'city' => 'New York',
  'name' => 'Bill'
 },
 {
  'city' => 'New York',
  'name' => 'Ronald'
 },
 {
  'city' => 'Boston',
  'name' => 'George'
 }
];

##</code><code>##

$VAR1 = {
          'New York' => [
                          'Bill',
                          'Ronald'
                        ],
          'Boston' => [
                        'George'
                      ]
        };

##</code><code>##

map { push @{$var1->{$_->{city}}}, $_->{name} } @{$var};