use strict;
use warnings;
use Data::Dumper;

my %city_data = ();
foreach my $filename  ( 'cp_123456', 'cp_123457') {
    my ( $city, $id ) = split /_/, $filename;
    open THIS, "<$filename" or die "Can't open $filename for reading: $!";
    chomp ( my @tmp = <THIS> ); 
    close THIS;
    push @{ $city_data{$city}{$id} }, @tmp;
}

print Dumper( \%city_data );