Dear Tyge,
since your question is about plain Tcl, it is probably better to ask this question on e.g. comp.lang.tcl.
A short answer is that if you want to call a proc from a script, the proc has to be defined first. One can load procs defined in a script by sourcing it [1] or by defining packages [2].
In the OpenACS-case, the API is preloaded from all the library files into a "blueprint" (containing all procs), which is made available to the requests and background jobs.
[1] https://www.tcl.tk/man/tcl8.5/tutorial/Tcl30.html
[2] https://www.tcl.tk/man/tcl8.5/tutorial/Tcl31.html
