While it would be easier with a separate Entry widget, you should be able to do with Tk::Text. I don't have anything offhand to show you, but this has been asked on http://groups.google.com, just search for "perl Tk inertion cursor".

The easiest would be to know where the cursor needs to be by line-position. For instance in your code:

 
$ta->mark(qw/ set insert 2.15 /);
MainLoop;
[download]

will do it.

But you could also figure out a tag scheme, where you tag your input boxes, and set insert to the tag point.