This module is used to discover what RBL's are listing a particular IP address.
It parallelizes requests for fast response.

An RBL, or Realtime Blackhole List, is a list of IP addresses meeting some
criteria such as involvement in Unsolicited Bulk Email. Each RBL has its own
criteria for addition and removal of addresses. If you want to block email or
other traffic to/from your network based on one or more RBL's, you should
carefully study the behavior of those RBL's before and during such blocking.

WWW: http://search.cpan.org/dist/Net-RBLClient/