My pet project in those days was writing a C++ X-Windows gui toolkit, and I took a lot of inspiration from Tk. Both design, and "how do I do this" from the source code were invaluable.

Didn't Cisco use Tcl in one of their IOSes? Found it: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ios_tcl/co...