$ua = LWP::UserAgent->new;

my $req = POST 'http://thesite/thepage.aspx', [ errors => 0 ];

$html = $ua->request($req)->as_string;

$html =~ /name="__VIEWSTATE" value="([^"]+)"/s;

$viewState = $1;

my $req = POST 'http://thesite/thepage.aspx',
[ __EVENTTARGET => 'theItemID', __EVENTARGUMENT => 'newValue', 
__VIEWSTATE => $viewState, errors => 0 ];

$html = $ua->request($req)->as_string;