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;
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