I imagine it needs to be encoded as JSON

No - Stripe takes key/value pairs.

To quote the documentation:
accepts form-encoded request bodies, returns JSON-encoded responses

For a form-encoded request, application/x-www-form-urlencoded is required hence the use of post_form().

The syntax of the call to Stripe works elsewhere in my code. Indeed, the call I am making works provided I don't make another call before it - it is this problem I want help resolving.

I'm unsure if it is an issue with HTTP::Tiny not being able to reuse the header information or an issue with Stripe or something else...

Out of interest, I have tried using the request() method as you suggested.
With the first call still in the code I continue to get a 401 error and the message that I have not supplied an API Key.
If I comment out the first call to Stripe, it accepts the API Key but returns the entire subscription object because it is not accepting the JSON encoded payload.

In reply to Re^2: HTTP::Tiny losing headers for Stripe by Bod
in thread HTTP::Tiny losing headers for Stripe by Bod

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