use WWW::Mechanize ;

my $m = WWW::Mechanize->new ;
my %conf = ( foo => bar ) ;

$m->get("http://your.url/") ;
die $m->res->status_line unless $m->success ;

# Choose form number
$m->form_number(1) ;

# Fill
$m->set_fields( %conf ) ;

# Submit
$m->submit ;
die $m->res->status_line unless $m->success ;

# If the form sends you somewhere, you can catch it :
my $url = $m->response->request->uri->as_string ;