Hi,

• The URL is not a named argument for HTTP::Tiny request methods. It's the first positional argument.
• Passing a Perl data structure as the content of the POST request won't work. You need to serialize it.

Try this with your real auth token:

$ perl -Mstrict -MJSON -MData::Dumper -MHTTP::Tiny -E'my $resp = HTTP:
+:Tiny->new->post("https://moneybird.com/api/v2/123/external_sales_inv
+oices", { headers => {Accept => "application/json", "Content-Type" =>
+ "application/json", Authorization => "Bearer 123"}, content => to_js
+on({ external_sales_invoices => { "reference" => "M-8426", "date" => 
+"2020-03-14", "contact_id" => 456, "details_attributes" => [ { "descr
+iption" => "coffee", "price" => 2.25, "tax_rate_id" => 891 } ] } })})
+; say Dumper $resp->{content}'
[download]

$VAR1 = '{"error":"access token invalid","symbolic":{"request":"invali
+d_grant"}}';
[download]

Hope this helps!