12.34.56.78/16|City A
90.12.34.56/19|City B
etc

##</code><code>##

# Build an array.  Each element of the array is a reference
# to a two-element array consisting of a Net::Netmask object
# and a city name
my @blocks;
while (<FH>)
{
    my ($block, $city) = split /\|/;
    push @blocks, [Net::Netmask->new($block), $city];
}

##</code><code>##

sub find_city
{
    my $address = shift;
    foreach my $arrayref (@blocks)
    {
        # Extra variables assigned for clarity
        my $netblock = $arrayref->[0];
        my $city = $arrayref->[1];

        if ($netblock->match($address))
        {
             return $city;
        }
    }
}

##</code><code>##

$blocks{10}{0} = [
   [Net::Netmask->new("10.0.0.0/24"), "City A"],
   [Net::Netmask->new("10.0.1.0/24"), "City B"]
];
$blocks{10}{1} = [
   [Net::Netmask->new("10.1.0.0/24"), "City C"],
   [Net::Netmask->new("10.1.1.0/24"), "City D"]
];