Better late than never, eh? This uses a hash to store all the ranges, has to do an explicit sort and so doesn't depend on the input being sorted. It runs in about 10 seconds on your nm5.in sample file (which is a good deal better than the 3 1/2 minutes it took me to run Net::CIDR and end up with non-optimal results). I haven't tried real hard to break it, and it doesn't check for valid data, so buyer beware.

There was a similar (generic) range merging script posted awhile ago here on PM that used a hash and an accumulating count, I can't find it at the moment or remember who posted it, but credit to them, whoever they may be :)

Updated with tye's suggestion from this node.

use strict;
use warnings;
use Socket qw(inet_ntoa inet_aton);

my @masks = map { pack("B*", substr("1" x $_ . "0" x 32, 0, 32))
                } 0..32;

my @bits2rng = map { 2**(32 - $_) } 0..32;
my %rng2bits = map { $bits2rng[$_] => $_ } 0..32;

my %ips;
while (<>) {
    my ($ip, $mask) = split "/";
    my $start = inet_aton($ip) & $masks[$mask];
    my $end = pack("N", unpack("N", $start) + $bits2rng[$mask]);
    $ips{$start}++;
    $ips{$end}--;
}

my ($start, $total);
for my $ip (sort keys %ips) {
    $start = $ip unless $total;
    unless ($total+=$ips{$ip}) {
        my $diff = unpack("N", $ip) - unpack("N", $start);
        while ($diff) {
            (my $zeros = unpack("B*", $start)) =~ s/^.*1//;
            my $range;
            for my $i (32-length($zeros)..32) {
                $range=$bits2rng[$i], last if $bits2rng[$i]<=$diff;
            }
            print inet_ntoa($start), "/", $rng2bits{$range}, "\n";
            $start = pack("N", unpack("N", $start)+$range);
            $diff -= $range;
        }
    }
}
[download]

Update: See also Net::CIDR::Lite ?? (Merge CIDR addresses).