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<FORM class="form_fonce" ACTION="/app-html/general/action.htm" NAME="f
+orm_machine" TARGET="indic_machine"
OnSubmit="return saisie_indic(document.form_machine.machine.value);">
...
<INPUT TYPE="button" NAME="decl"         VALUE="Coll"
OnClick="return coll(document.form_machine.machine.value); ">
</FORM>

my $url = 'http:/app/correct/path/form_machine.htm' ;
my $mech = WWW::Mechanize->new();
...
$mech->form_name('form_machine') ;
$mech->field( 'machine', $machine ) ;
$mech -> click('IwantThisOne') ;

No clickable input with name IwantThisOne