@Corion, thanks for the suggestion. I've now recorded the headers sent to the server by Firefox using Live HTTP headers. They are something like:

Host: 119.167.217.206:19765
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN; rv:1.9.1.1
+1) Gecko/20100701 Firefox/3.5.11
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.
+8
Accept-Language: zh-cn,zh;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: GB2312,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive

HTTP/1.0 200 OK
Content-Type: video/x-flv
Content-Length: 21293994
Connection: close
[download]

As you suggested, I've also tried to send the same headers in Mechanize:

use WWW::Mechanize;
use strict;
use warnings;

my $browser = WWW::Mechanize->new();
$browser->cookie_jar(HTTP::Cookies->new()); 
$browser->add_header('User-Agent' => 'Mozilla/5.0 (Windows; U; Windows
+ NT 5.1; zh-CN; rv:1.9.1.11) Gecko/20100701 Firefox/3.5.11');
$browser->add_header('Accept' => 'text/xml,application/xml,application
+/xhtml+xml;q=0.9,*/*;q=0.8');
$browser->add_header('Accept-Language' => 'zh-cn,zh;q=0.5');
$browser->add_header('Accept-Encoding' => 'gzip,deflate');
$browser->add_header('Accept-Charset' => 'GB2312,utf-8;q=0.7,*;q=0.7')
+;
$browser->add_header('Keep-Alive' => 300);
$browser->add_header('Connection' => 'keep-alive');
my $url = 'http://119.167.217.206:19765/ppvaplaybyopen?url=http://119.
+167.217.206/%d3%e9%c0%d6%b0%d9%b7%d6%b0%d9-100803-%d0%a1%d6%ed%bd%dc%
+c2%d7%b7%d6%d7%e9%d0%e3%c7%f2%bc%bc.mp4/segno=0%26&rid=A8F1F5DFEB1B11
+F1D90B40AD1BB75D69&filelength=21293994&blocksize=2097152&blocknum=11&
+blockmd5=E210862B3F92935D0883E00AA2A38F08@D793599727C6DA4ACDB1CBF2235
+004AC@D5E9C9245C9A1BB63BC5EDA862A32604@51B5FDF91356B2B4E943EF72648EB0
+AD@6F2400488B04EBF66A60336B795EA142@8E51B8DCF87A7A02B84A2CAA5FFCA3CF@
+89080D683268481694DBA6D1E22A2EFF@8F56225C76854A434385A09C319BF9C3@9AB
+0A3F199183F479F8887D1C3341B1B@845FE0D711086CC2D086546CD26B35C1@9D93A9
+BE1D2EDE216AA9EBF26BF414BE';

$browser->get($url);
[download]

But I'm receiving the same error as follows:

Error GETing http://119.167.217.206:19765/ppvaplaybyopen?url=http://11
+9.167.217.
206/%d3%e9%c0%d6%b0%d9%b7%d6%b0%d9-100803-%d0%a1%d6%ed%bd%dc%c2%d7%b7%
+d6%d7%e9%d
0%e3%c7%f2%bc%bc.mp4/segno=0%26&rid=A8F1F5DFEB1B11F1D90B40AD1BB75D69&f
+ilelength=
21293994&blocksize=2097152&blocknum=11&blockmd5=E210862B3F92935D0883E0
+0AA2A38F08
@D793599727C6DA4ACDB1CBF2235004AC@D5E9C9245C9A1BB63BC5EDA862A32604@51B
+5FDF91356B
2B4E943EF72648EB0AD@6F2400488B04EBF66A60336B795EA142@8E51B8DCF87A7A02B
+84A2CAA5FF
CA3CF@89080D683268481694DBA6D1E22A2EFF@8F56225C76854A434385A09C319BF9C
+3@9AB0A3F1
99183F479F8887D1C3341B1B@845FE0D711086CC2D086546CD26B35C1@9D93A9BE1D2E
+DE216AA9EB
F26BF414BE: Internal Server Error at E:\pp2.pl line 17
[download]

Is there anything else I can try? Thanks :)