Now I have a higher state of enlightenment.

The only reason it worked before was a fluke. For form-data I would need  content => [ @query ] but I was sending in  content => $query , which requires x-www-form-urlencoded. Luckily for me, content-type is a sum which yields zero, so the header said 0 => form-data and the default content-type was put in.

Aaah! Said Stork.

I will be more careful in future and use strict from the start!