Tk::TipEntry
Perl/Tk widget for an entry with tooltip in the entry if it's empty
...The tooltip will be removed, if the entry gets the focus and reinserted, if the entry loses the focus and it's value is empty.
In addition, the entry evaluates the escape-key. If the entry has the focus and the escape-key is pressed, the original input will be restored. If there is no previous input, the tooltip will be displayed again.