I used Tcl at my very first internship for test automation. Coming from a self-taught Javascript/Java/C#/C++ background, Tcl really stretched my concept of what a programming language is, especially how it plays with stack frames and munges string values.

Coincidentally Tcl also inspired my first deep dive into Vim. The test automation framework at that company required meaty Tcl expressions packed into CSV cells. Tcl was mind-boggling enough when properly formatted, so hand-editing squashed 1800 character long expressions on a single line was way too much. I'm fairly certain the assignment was part hazing and the framework part "job security".

Jokes on them, I spent 2 weeks on a side quest to make a Vim plugin that could add line breaks and indent a Tcl expression into a split temporary buffer for editing and squash it back to the right cell when I was done. Halfway through the summer I had completed the entire backlog. The manager got a reprimand for being a jerk and I spent the rest of the summer making the framework less painful.