Thank you for the "monkey patching" hint, used it and the solution to make text editable is quite simple:

use Tk::TextUndo;
use Tie::Tk::Text;
sub Tk::Wizard::_my_text_frame {
#...
 my $t = $frame->Scrolled(
  "TextUndo",
 )->pack;
 if ( $args->{-variable} ) {
  tie @{$args->{-variable}}, 'Tie::Tk::Text', $t;
 }
#...
}
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which can be called as

$wiz->_my_text_frame(
 {
   -title     => 'editable text',
   -boxedtext => \$originaltext,
   -variable => \@editedtext
 }
);
[download]