Well, mine isnīt in Perl, C/C++, etc. but I like it ;-) It is written in JavaScript (anybody who says ECMAScript? ;-) and is a generic HTML form validator which lets HTML designers use nothing except their HTML skills to create forms that validate themselves.
The idea is to use the utility of user-defined-HTML-attributes-on-the-fly and thus get rid of tackling with JScript at all.
The idea is explained (only in Turkish, if anybody is interested I can translate it into English) here:
Generic HTML Form Validation You can download the JS package and the sample web page. It is compatible with Mozilla, Netscape, Internet Explorer. The difference between this system and the others is that they need the programmer/designer to modify the JavaScript in one way or the other.
I'm sure Perl has got nice solutions to this frequently encountered problem but in case you donīt use Perl in every Internet project, I think this tool can help you in a valuable way ;-)