See Tcl Improvement Proposal, if you are looking for the mechanism by which changes to Tcl and Tk are introduced, accepted, and documented.
Long ago, when "Unix" often as not meant "Solaris" (except that it was called "SunOS" then) or something now regarded as even more recondite, serial port work generally involved the command-line utilities tip(1) and cu(1) ([L1 ], or [L2 ], for example). Questions sometimes still arise about these from new readers of Exploring Expect, because that venerable volume was written at a time when tip(1) constituted a natural example for its lessons. tip(1) and cu(1) functions now might be done by ckermit [L3 ], minicom [L4 ], ...
[Was tip(1) specifically a BSD creation?]