https://www.fossil-scm.org/xfer/doc/trunk/www/th1.md
It's a tiny Tcl subset, implemented in only a few kloc. It's used by Fossil as a HTML templating engine but it's easily adaptable for other things. It's small enough to be understood and does an excellent job of demonstrating the core principles of how string-processing languages think.
drh is a former Tcl Core Team member[0], and probably most famous as the author of Sqlite, originally a Tcl extension that "escaped into the wild"[1]. Sqlite also has a comprehensive test-suite written in Tcl[2].
[edit: footnotes]
Tiny Tcl implementations are fun in general and perhaps easier to produce than tiny Scheme implementations — as long as you are willing to concede that everything really is a string. :-) (I.e., no caching binary representations; you have to parse from scratch each time.) In a similar vein as TH1 but with a little more functionality there are Picol (https://tcl.wiki/Picol) and LIL (http://runtimeterror.com/tech/lil/).
Disclosure: I maintain an expanded fork of Picol. The original version of it written by antirez was only ~550 LOC but with suchenwi's additions and mine it is now around 3100. There is a link to it on the wiki page. The change that I am most fond of is making it an stb-style (https://github.com/nothings/stb/) header library.
Oh, I agree with you on that. My point was that Tcl makes it especially easy to "cheat" and avoid building an AST. You may do this when you only care about writing an interpreter in few LOC, like the original version of Picol, quickly.
A Tcl interpreter in VHDL sounds interesting. Do you have a public repository for it?
ps: th.c is one fun name I failed to anticipate.