I'm a big fan of Net::Netmask. I maintain a simple database of my company's netblocks and which city they belong in by building an array of Net::Netmask/city combinations. Assuming the netblocks/cities are stored in a pipe-delimited text file like so:

12.34.56.78/16|City A
90.12.34.56/19|City B
etc
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The code looks something like this:

# 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];
}
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Then to check which city an ip address belongs in, I just do a sequential search:

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;
        }
    }
}
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The code could be written faster by using a better search algorithm [1], but a sequential search of my company's 2100+ netblocks takes negligible time so I haven't seen fit to recode it.

[1] We don't have anything larger than a /16, so hash buckets keyed on the first two octets could speed things up a bit. Something like this:

$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"]
];
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-Matt