Your error message consists of two concatenated strings:
Impossible d'ouvrir le formulaire: - You wrote this, so it would appear that $access_obj->DoCmd->OpenForm('Formulaire1') was not successful. As you used warn, your code continued but possibly with bogus -- undefined, empty, etc. -- data.
You also wrote $! which interpolated as "No such file or directory". This error is often resolved by supplying a relative ../Formulaire1 or absolute /some/path/to/Formulaire1 pathname. Furthermore, your OS may ignore filename extensions then act dumb when you supply it with exactly the same filename strings that it supplies to you: you may need to specify Formulaire1 as Formulaire1.XXX (where XXX is the hidden extension).