For example, I'd never use XML::XPath in any project that used a largish XML file and needed to get done somewhere near quickly... but XML::LibXML works great, and still allows me to do my work in Perl.

Usually where the projects are going to fail is where speed becomes an issue, or where Perl isn't available, or memory issues. For the speed most people turn to wrapping C, the other two are a little tougher :)