__RequestVerificationToken: acP92nszWSeNIAeL/Al9Rj24scVQXt+o

##</code><code>##

my $mech = WWW::Mechanize->new;
my $r = HTTP::Request->new("POST", $some_url);
$r->header("__RequestVerificationToken" => $key);
# set content and some other headers here
$mech->request($r);

##</code><code>##

--RequestVerificationToken: whatever