You can also use indices ( see the section on Indices) in "perldoc Tk::Text". Essentially, you find the start indice of your word, count the number of characters in the word, delete ($indice + $n chars), the insert at the same $indice.

If you show a working code example, we can probably show you how.

When using 'startline.end' there is a Tk Error that the index is bad

Then maybe 'startline' is nothing legal. I only found 'linestart' in the docu. 'end linestart' could actually work to mean the start of the last line in a text, if I read the documentation correctly

If not, try something like '1.0' or 'x.0' with x the line number of the line where your filesize is put in the text widget

Also note that there is a spelling error in the perldoc for Tk::Text, deleteTextWithTag should be DeleteTextWithTag.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $mw = MainWindow->new;
my $update = 0;

$mw->fontCreate('big',
   -family=>'arial',
  -weight=>'bold',
  -size => 14 );

my $text = $mw->Text(-font=>'big')->pack;

$text->tagConfigure(
    'updatetag',
    -borderwidth => 3,
    -background  => 'yellow',
    -relief      => 'raised',
);


my $string = join ' ',('a'..'z');
foreach my $line(0..10){
     $text->insert('end',"$line->$string\n");
  }


$text->delete("7.11", '7.13');
$text->insert("7.11", "$update ",['updatetag']);

my $bFrame = $mw->Frame->pack(qw/-side bottom/);

$bFrame->Button(
   -text => "Update Text",
   -command => sub {
      $update++;

#  this works, but adds spaces as $update gains characters
#      $text->delete("7.11", '7.13');
#      $text->insert("7.11", "$update ",['updatetag']);

#works good
      $text->DeleteTextTaggedWith('updatetag');
      $text->insert("7.11", "$update ",['updatetag']);
  
})->pack(qw/-side left/);

MainLoop;
[download]

Sorry for forgetting this info - Tk

there is an invisible newline at the end of a Text widgets content. - Plus possibly one as part of the last line you inserted?

use warnings;
use strict;
use Tk;

my $mw = tkinit;
my $text_log = $mw->Text->pack;
$text_log->insert(end => "test\n") for (0..2);
my $i = 0;
$mw->repeat(500,
            sub{
                $text_log->delete('end -2line linestart', 'end -1line'
+);
                $text_log->insert('end', "Current Size:$i\n");
                $i++;
            });
MainLoop;
[download]

Thanks guys! It works!

I've implemented lamprecht method and thanks, zentare, for itroducing to me the 'taged' way