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

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

sub find_city
{
...
        }
    }
}

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