It largely depends on what you want to do with the data...are you trying to construct a summary of the digits based on the city keys? In that case I would create a hash of arrayrefs and then manipulate the data as you need:

while(<DATA>){
 ($city,$num)=split;
 push( @{$myhash{$city}}, $num );
}
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Now you have a hash with the city names as keys and each corresponding digit included in an arrayref of values. This node has some good insight into working with these data structures.