You might be aware that there are two ways of executing IOS CLI commands from IOS tclsh: either you insert the IOS commands directly in the Tcl code (tclsh passes any unknown commands to the CLI parser) or you execute them with the exec command. There are subtle differences between the two methods, described in the “Executing IOS commands from Tcl shell article” in the CT3 wiki.
Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.