You are reading the entire file in that loop: 
while (<$fh>) {
    $st->insert("end", $_);
}

[download]

You probably want 

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');
}

[download]

Roughly.

--shmem 

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

In reply to Re: insert in Text widget by shmem
in thread insert in Text widget by xiaoyafeng

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.