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&__EVENTARGUMENT=newValue&__VIEWSTATE=$viewState");
}