while (<$fh>) {
    $st->insert("end", $_);
}

##</code><code>##

use strict;
use warnings;

use Tk;

my $fh;
open ($fh, "<D:\\perl_script\\sqlnet.log") or die "error!!$!\n";

my $mw = MainWindow->new();
my $st = $mw->Scrolled("Text",-scrollbars => "eo")->pack();
my $id = $st->repeat(3000,\&list);
MainLoop;
close ($fh);

sub list {
    if (eof($fh)) {
        $st->aftercancel($id);
        return;
    }
    my $line = <$fh>;
    $st->insert('end', $line);
    $st->see('end');
}