Lua is simple to bolt on to your project.

Perl is a nightmare to interop! They should have won an award for that. It took effort.

Python is slightly cleaner than perl, but not much.

You mean, the interpreter is available as a library with bindings in many languages?

But then - isn't that the case for other scripting languages these days? Or are they too complex and with "strings attached"?

I can't speak for other scripting languages, but it's absurdly easy both to embed Tcl as an interpreter in a larger project, and to create a Tcl extension which implements your own commands.

You're using the same words. What is "easy" about embedding TCL that is "difficult" about embedding Perl or Python? Can you spell this out?

Sorry, missed this at the time. But I did say I can't speak for other scripting languages. All I can say is that it was much easier than I was expecting!

One can leverage Lua in PHP, I think it's handy for user-supplied logic (eg: business rules)

I like TCL a lot more for this use case, and I don’t care for Lua at all; but Lua might be a better match for embedding in an OO language.