Well, my bot does it something like this:

print "Connecting to Mastodon...\n";
my $client = Mastodon::Client->new(
    instance        => $config->{server},
    name            => 'TotallyLegitBot',
    client_id       => $config->{key},
    client_secret   => $config->{secret},
    access_token    => $config->{token},
    coerce_entities => 1,
);

print "Posting message...\n";
$client->post_status('Dear Sir! I am prince Bob of Nigeria and i have 
+an offer for you...');
[download]

Hope that helps.

EDIT: For authentication, i went to my Mastodon account to Preferences/Development, clicked on "New Application", filled in application name and clicked submit. Then click the link with that application again and you get key+secret+token that you give your application. There are probably other ways to authorize it, but that's the one i found works without problems and with the least fuzzing about.