hmm ... I tried :

$ie->form_name('form_machine') ;
$ie->field( 'machine', $machine ) ;
$ie -> click('IWantThisOne') ;
[download]

This has the effect of oppening an IE window with the page i want ! And that's with visible => 0 at the creation of the IE object ?! With visible => 1 it doesn't open the correct page ...

I'll bypass the problem by attacking the URL directly but it's a little bit disappointing ...