$text->delete($start_index,$end_index);
$text->insert($start_index,$my_new_text);

##</code><code>##

use strict; 
use Tk;

my $mw = MainWindow->new();

my $frame = $mw->Frame()
    ->pack(-side => "left", 
	   -expand => 1,
	   -fill => 'both');

my $text = $frame->Scrolled('Text')
    ->pack(-expand => 1,
	   -fill => 'both');

my $data;
for (my $i = 1; $i < 20; $i++) {
    $data .= "abcdefghi $i \n";
}
$text->insert('end',$data);

my $but1 = $frame->Button(-text => "Replace in the middle",
			  -command => [\&replace,$text,'3.0','7.0'])
    ->pack(-side => 'left');

my $but2 = $frame->Button(-text => "Replace in the end",
			  -command => [\&replace,$text,'16.0','end'])
    ->pack(-side => 'left');

MainLoop(); 

sub replace {
    my ($text,$start_index,$end_index) = @_;
    my $old_data = $text->get($start_index,$end_index);
    $old_data =~ s/cdefg/12345/g;
    $text->delete($start_index,$end_index);
    $text->insert($start_index,$old_data);
    return;
}