Script doesn't return any error,but torrent isn't uploaded.

I haven't used WWW::Mechanize much but the first thing that leaps out at me is you are not doing any sort of error detection within the script. There are a couple of methods I'd suggest you read up on for the module the first being  success() and the other being status()

$mech->get("http://localhost/TB_ALPHA/upload.php");
sleep 2;
warn "wait";
$mech->field('file', $torrent_file); #input torrent file
warn "wait $torrent_file";
$mech->field('name', $name); #file name
warn "wait $name";
$mech->field('descr', $descr); #description
warn "wait $descr";
$mech->value(3); #category
warn "wait";
$mech->click_button(value => "Do it!"); #Clicks do it
warn "wait";
[download]

That bit there makes me suspicious as well. I don't think it is doing what you think it is...