I've never gotten around to doing this programaticaly, but I suspect that one of the following modules (or a combination) might maybe do what you're looking for.   I've sorted the following list with what looks to be more promising modules first:

• Net::CIDR
• Net::CIDR::Lite
• Net::Netmask
• Tie::NetAddr::IP
• Net::IP
• Net::IP::Match
• Net::ACL
• Net::IPv4Addr
• Network::IPv4Addr
• NetAddr::IP
• Net::IP::Match
• NetAddr::IP::Count
• Net::Subnet::Count
• CIDR::Assign

Wish I could provide more concrete help, but I hope this is beneficial in some small way.   %^P
  cheers,
  ybiC

  striving toward Perl Adept
  (it's pronounced "why-bick")

Update: Fwiw, I found these modules through http://search.cpan.org using these terms: netmask, subnet, ip address, and cidr.

Update II: Anonymonk provides below what was beginning to percolate into my brain - that is, convert the dotted quad addresses to their (real) binary values, then sort and look for sequential blocks.   ++anonymonk.