or download this
$ ack EnterFocus ~/perl5/lib/perl5/
/home/choroba/perl5/lib/perl5/x86_64-linux-thread-multi/Tk.pm
...
/home/choroba/perl5/lib/perl5/x86_64-linux-thread-multi/auto/Tk/focusF
+ollowsMouse.al
10: $widget->bind('all','<Enter>','EnterFocus');