Artifact b605e70175e374375e29e148cd61092a83496e2554836aba8dffcfd9ab1cb2c2:
#!/usr/bin/tclsh
package require Tk
set ::var "Hello World!"
proc tproc args {
.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]