Artifact e521e736d785900bf24d2603b5b3725efc7c82686e028a1708a284b172bfbe0c:
#!/usr/bin/tclsh
package require Tk
set ::var "Hello World!"
proc tproc args {
set var "Boom!"
.b configure -textvariable {} -bg red
}
# order is important
button .b -textvariable ::var
trace add variable ::var {write unset} tproc
pack .b
after 2000 [list unset var]