As your code grows, you'll want to divide it into series of modules. These are known as packages in Tcl. You'll also want to use a separate namespace so that procedure names in one package don't conflict with the same procedure name in another. Libraries allow you to load code in C as well as Tcl into your interpreter.

This topic is presented on public course Tcl Basics

ns_exim	Namespace export and import
ns_no	shows how variables are lost between proc calls
ns_up	Variables saved between proc calls in the next code level up
ns_y2	Counting proc calls using a namespace - shorter example
ns_yes	Saving variables in a namespace
packs	Loading a package
show.tcl	Defining a package

creating packages in Expect
How to use global variable in one namespace
Public Tcl course