alright with the following code I got it to work.
Thanks! 
sub create_external_inv {
    my $agent   = HTTP::Tiny->new;
    my $headers = {
        Accept         => "application/json",
        'Content-Type' => "application/json",
        Authorization =>
"Bearer 123"
    };

    my $url =
      "https://moneybird.com/api/v2/123/external_sales_invoices";

    my $data = {
        external_sales_invoice => {
            "reference"          => "M-8426",
            "date"               => "2020-03-14",
            "contact_id"         => 456,
            "details_attributes" => [
                {
                    "description"       => "coffee",
                    "price"             => 2.25,
                    "tax_rate_id"       => 891,
                }
            ]
        }
    };

    my $res =
      $agent->post(
        $url => { content => to_json($data), headers => $headers } );

    print Dumper $res;
    if ( $res->{success} ) {
        print "Okay!\n";
    }
    else {
        print "\n\nNope...\n\n";
    }
}

[download]
The results print okay!.

In reply to Re^3: post JSON using HTTP::Tiny - cannot parse URL by GertMT
in thread post JSON using HTTP::Tiny - cannot parse URL by GertMT

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