ddclient is the name of one Perl script that they use-- it's pretty straightforward. It's got nice configuration files that you can use to specify multiple domain names, and use it for multiple domain name services, not just DynDNS.
Registered Linux user #230403! Since March 2001! YAY.
Try doing a forum search or a google search before asking a question. And please don't use HELP! in the topic of your post... it's so lame... Please don't PM me for help-- post a question in the forum instead.