sub xyz{
  $wid->repeat(1, sub{ $var=1; } ); # after 1 millisecond $var==1
  # 1 ms... 2ms... 3ms...
}

{
  &xyz;
  # 6 ms... 7ms... 8ms...
  $widget->waitVariable(\$var); # will this wait indefinitely because the var was already changed???
  # Continue... never will happen
}