I still work on a TCL codebase. I hate it. It makes heavy use of upvars, uplevels, and even a DSL written in TCL. It's so limiting. The interpreter only uses a single thread so better hope you don't need to do much computing with it. It's so hard to debug. Oh and be careful not to use brackets in your comments cos it interprets comments as if they're code...

The codebase is too big to rewrite but we're slowly untangling and hacking bits off to write in golang, but most of the business logic is in that DSL.