@TCL[expression] : Returns the string result of the Tcl expression. (You cannot run a Tk script in @TCL, because there is no Tk event loop. If you want to run a Tk script, you need to execute it from the command line as you would a Tcl script, or with the @TK function.)
The Tcl interpreter in TCC is persistent, so if you want to reset it pass an empty string to @TCL.
See also @TK.