HTML::Mason is a full-featured web site development and delivery system. Mason allows web pages and sites to be constructed from shared, reusable building blocks called components. Components contain a mix of Perl and HTML, and can call each other and pass values back and forth like subroutines. Components increase modularity and eliminate repetitive work. HTML::Mason is most powerful when used in conjunction with mod_perl, but can be used in standalone applications as well.
txt2html is a Perl program that uses the HTML::TextToHTML perl module to convert plain text to HTML. It supports headings, lists, tables, simple character markup, hyperlinking and is highly customizable. It recognizes some of the apparent structure of the source document (mostly whitespace and typographic layout) and attempts to mark that structure explicitly using HTML.
The HTML::Defaultify Perl module is designed to allow CGI programmers to present forms with some fields filled in, which is handy for letting users correct bad input, show records to be edited, and much more. The main function, defaultify(), takes any block of HTML and a hash of field names and values, and returns the HTML with the form fields set to those values. Value lists for multiple same-named fields are supported, as are pages with multiple forms. Several different input formats are supported for compatibility with existing tools.
This module implements an XML::SAX filter which adds line numbers and column numbers to errors generated by SAX handlers further down the pipeline. It was written so that XML::Validator::Schema could blame the correct line of XML for validation failures. You can use this module to enhance the error messages generated by any SAX handler.
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.