Most of the time a GET request works too for form submission, so you might wanna try something like this:

use LWP::Simple;

my $baseURL = "http://thesite/thepage.aspx";
my page = get($baseURL);

if ($page =~ m/name="__VIEWSTATE" value="([^"]+)"/s)
{
  my $viewState = $1;
  my $submitResult = get("$baseURL?__EVENTTARGET=theItemID&__EVENTARGU
+MENT=newValue&__VIEWSTATE=$viewState");
}
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Warning: untested code.