use HTTP::Headers;

my $h = HTTP::Headers->new();

$h->header('Content-Length' => 61 );
$h->header('Content-Type' => 'application/x-www-form-urlencod
ed');

my $request = HTTP::Request->new( 'POST', 'http://www.perl.org/survey.cgi', $h );

my $response = $ua->request( $request );
...