Final version:

#!/usr/local/bin/perl
use Tk;
use Tk ':variables';

use strict;
use warnings;

my @choices = qw/seeing something important reply reduction renowation
+ reconnaissance/;


# Main Window
my $mw = new MainWindow;

#GUI Building Area
my $frm_name = $mw -> Frame();

#Text Area
my $textarea = $mw -> Frame();
my $txt = $textarea -> Text(-width=>40, -height=>10);
my $srl_y = $textarea -> Scrollbar(-orient=>'v',-command=>[yview => $t
+xt]);
my $srl_x = $textarea -> Scrollbar(-orient=>'h',-command=>[xview => $t
+xt]);
$txt -> configure(-yscrollcommand=>['set', $srl_y], -xscrollcommand=>[
+'set',$srl_x]);

my $log = $textarea -> Text(-width=>40, -height=>2);

#Geometry Management
$frm_name -> grid(-row=>1,-column=>1,-columnspan=>2);

$txt -> grid(-row=>1,-column=>1);
$log -> grid(-row=>6,-column=>1);
$srl_y -> grid(-row=>1,-column=>2,-sticky=>"ns");
$srl_x -> grid(-row=>2,-column=>1,-sticky=>"ew");
$textarea -> grid(-row=>5,-column=>1,-columnspan=>2);

$txt->bind('<KeyPress>', [\&OnKeyPress, $txt]);

$txt->focus;

MainLoop;

sub OnKeyPress
{
    my $txtBox = $_[0];
    
    my $currentWord = $txtBox->get('insert -1c wordstart', 'insert');
    my $wordEnd = $txtBox->get('insert', 'insert -1c wordend');
    $log->delete('0.0', 'end');
    $log->insert('end', "wordEnd: <$currentWord, $wordEnd>");

    # do not want autocompletion in the middle of the word
    if(length $wordEnd == 0
    # want autocomplete only when typing, not on key up, down, enter, 
+etc.
    && $Tk::event->K =~/^[\w]$/)
    {
        for(@choices)
        {
            if(/^($currentWord)(.*)/)
            {
                my $theRest = $2;
                my $len = length $theRest;
                my $selFrom = $txtBox->index("insert");
                $txtBox->insert('insert', $theRest);
                $txtBox->tag('add', 'sel', "insert - $len chars", "ins
+ert");
                
                $log->insert('end', "matched: <$theRest>");
                last;
            }
        }
    }    
    
    return;
}
[download]

It took a while searching Google and testing variations of SelectFrom(), SelectTo(), tagging, etc. Until I found tag('add', 'sel', "insert - $len chars", "insert") in sources of Tk::SuperText and voila! I still don't know why this code doesn't work without '-1c', but it seems to be a good start for creating crystal ball.