XML::Parser is a Perl extension module providing an interface to James Clark's fast and robust XML parsing library, expat. Other Perl XML modules (many based on XML::Parser) may be found at the same CPAN download site where XML::Parser is located. The expat library is distributed as part of the module.
XML::Twig is an XML transformation module. It can be used to process huge documents while still being in tree mode; is not bound by DOM or SAX, so it is very Perlish and offers a very comprehensive set of methods; is simple to use; and DWIMs as much as possible. What it doesn't offer: full SAX support (it can export SAX, but only reads XML); full XPath support (unless you use XML::Twig::XPath); nor DOM support. It is also a big module, and with over 500 methods available it can be a bit overwhelming. A good starting point is the online tutorial.
XML::XPath locates nodes in XML documents using the XPath syntax. It aims to fully comply with the specification while trying to be as quick as possible. It is also extensible for XSLT and XPointer. It is entirely Perl-based to minimize dependencies and ease installation, and does not use XML::DOM, although it can build DOM trees using a SAX event driver.
Xapian is a search engine library, scalable to collections containing hundreds of millions of documents. It's written in C++ with bindings for Perl, Python, PHP, Java, Tcl, C#, Ruby, and Lua. It is a highly adaptable toolkit that allows developers to easily add advanced indexing and search facilities to their own applications. It supports the Probabilistic Information Retrieval model and also a rich set of boolean query operators. Omega is a Web search application built upon the Xapian library. It can index a Web server's document tree (including HTML, PDF, OpenOffice, MS Word/Excel/Powerpoint/Works, WordPerfect, RTF, PS, etc.), or data exported from arbitrary sources (e.g. SQL databases).
Zero RRD Framework provides a central HTTP-based service for importing into standard RRD databases and for generating graphs. A lightweight, easily extendable agent for the data sources pushes updates with minimal resource consumption to the master service using HTTP. With the introduction of VMs for the service, it's easy to test-drive the application and even the architecture (connecting agents to the service in the VM).
Zoidberg (a.k.a. zoid) provides a modular Perl shell written, configured, and operated entirely in Perl. It aspires to be a fully operational login shell with all the features one normally expects. But it also gives direct access to Perl objects and data structures from the command line, and allows you to run Perl code within the scope of your command line.
ZoneMaster is a tool for zone file and name server management. It consists of a library that can configure BIND and a Web-based user interface that is used to access the data that is stored in a database. A bootstrap script allows you to initialize the database with all necessary information retrieved from an existing configuration. All modifications are saved as jobs in the database, and a job handler makes the necessary changes from time to time. The user interface is based on XSLT-stylesheets and can be easily modified.
cgi_buffer is a group of libraries that may be used to improve performance of CGI scripts (and other content generation engines) in some circumstances by applying performance-enhancing HTTP mechanisms that are typically not supported by them. Currently, Perl, Python, and PHP4 are supported. The Python library may also be used as a wrapper around another CGI script.