The term infrastructure has a variety of meanings, depending on its use.

For instance, one might consider things like C compilers, assemblers, link editors, etc. to be a part of the infrastructure of a software development environment.

Then, for an application developer, tcl, perl, java, along with their extensions/modules/classes, etc. might be considered a part of the infrastructure - they are the tools which make it possible to build the next layer.

A recent article by Dan Bricklin calls software developers to take a fresh look at software development - to begin to think about developing a software infrastructure that can last for hundreds of years, rather than a dozen years. Read for his argument.

For the Tcl core, one might consider its infrastructure to consist of text editors, c compilers, and a few tools used for installation purposes.