After checking

https://www.yammer.com/api_doc.html#resources_messages_manipulating

When posting a new message, the response body will include the new message formatted as in message polling above. This allows you to immediately display the newly-posted message back to the user.

I think its a bug that the api_doc doesn't explicitly list what type of status code the response should have, like it does for many other methods.

I think you definitely need to check the content, maybe as per LWP::Debug?

$app->{browser}->add_handler("response_done", sub { shift->dump; retur
+n });
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Or maybe with

->SUPER::new(
    tokens => \%tokens, 
    browser => WWW::Mechanize->new( autocheck => 0 ),
)
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and then you can

  my $response = eval { $app->view_restricted_resource ... };
  unless( $response ){
    warn "ERROR $@ ";
    $response = $app->res; # WWW::Mechanize cache it for you
  }
  print  $response->content;
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or something like that