Tulip is a minimal Lisp-like language written in Perl. It is slightly less horrifying than you'd expect.
I wrote it to gain a better understanding of Lisp.
It is available under the new BSD license. You can download the source code from here.
In the process of writing it, I used POD extensively to document the internals. When I finished, I realized that I could create high-quality internal documentation simply by using a simple script to turn the code into POD items as well and then running the result through pod2html, giving me a kind of poor man's literate programming. The resulting document is here.
The name "Tulip" is a rough abbreviation for "Tiny Lisp in Perl".