In the spirit of there is more than one way to do it, here is my solution to the problem without using any of the CPAN modules. Just straight forward divide and conquer.

use strict;

chomp(my @CIDR = <DATA>);
foreach (@CIDR)
{
  if (/\//) {
    # expand the address list
    my ($base, $range) = /(\d+\.\d+\.\d+\.)(.*)/;
    my ($r0, $r1) = split /\//, $range;
    if ($r1 < $r0) {
        die "invalid ip range" if length($r1) >= length($r0);
        $r1 = substr($r0, 0, length($r0) - length($r1)) . $r1;
    }
    print "$base$_\n" for $r0 .. $r1;
  } elsif (/-/) {
    my ($base, $from, $to) = /(\d+\.\d+\.\d+\.)(\d+)-\d+\.\d+\.\d+\.(\
+d+)/;
    print "$base$_\n" for $from .. $to;
  } else {
    # singular IP address
    print "$_\n";
  }
}

__DATA__
192.168.0.1
172.18.0.0/5
172.19.1.0/4
172.20.2.0/8
172.21.3.128/130
100.0.0.1-100.0.0.3
172.21.3.128/110
[download]

And the output (last entry has incorrect range) -

192.168.0.1
172.18.0.0
172.18.0.1
172.18.0.2
172.18.0.3
172.18.0.4
172.18.0.5
172.19.1.0
172.19.1.1
172.19.1.2
172.19.1.3
172.19.1.4
172.20.2.0
172.20.2.1
172.20.2.2
172.20.2.3
172.20.2.4
172.20.2.5
172.20.2.6
172.20.2.7
172.20.2.8
172.21.3.128
172.21.3.129
172.21.3.130
100.0.0.1
100.0.0.2
100.0.0.3
invalid ip range at p20.pl line 12, <DATA> line 7.
[download]

Update: added simple IP - IP processing for smackdab.