my %Data;
while(<DATA>)
{
  my ( $City, $Value ) = split;
  push ( @{$Data{ $City }}, $Value );
}
__DATA__
amsterdam   23
amsterdam   88
london      65
ny          10
ny          16
la          21
la          65 
miami        9 
lapaz       10

##</code><code>##

(
  amsterdam => [23, 88],
  london => [65],
  ny => [10, 16],
  la => [21, 65],
  miami => [9],
  lapaz => [10]
)