Looking at this and considering my own post, I thought I would try to improve it. The following code allows trailing zeros to be left off the input CIDR and it will leave off the trailing zeros in the output. It also handles 0/0, which neither of our posts did

use strict;

sub cidr2bits {
  my $cidr = shift;
  my $n = $cidr =~ s,/(\d+)$, ? $1 : 32;
  my @n = $cidr =~ m,\d+,g;
  substr(unpack("B*",pack("C4", @n,0,0,0,0)),0,$n);
}

sub bits2cidr {
  my $bits = shift;
  my $n = length $bits;
  $bits .= "0" x 8;
  join(".", unpack("C*", pack("B*",($bits =~ /^((?:.{8})+?)0*$/)[0])))
+."/$n";
}

sub mergecidr {
  local $_ = join("\n", sort  map { cidr2bits($_) } @_);
  1 while s/^(\d*)\n\1.*$/$1/mg || s/^(\d*)0\n\1.$/$1/mg;
  map { bits2cidr($_) } (!@_ || length($_)) ? split : '';
}
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Update: Swapped the two s/// in the while to prevent the problem merlyn described. Now in the case of

10010
100100
100101
[download]

the first two lines will be merged first and yeild the correct result