Hi Jules,
I was just thinking about this and how when there are changes to the Strava application they are not reflected in the API docs or changelog.
Anyway, grumble over!
Perceived exertion is revealed in the detailed activity endpoint:
https://www.strava.com/api/v3/activities/{id} - though not in the summary activity endpoint, my app uses only the summary activity endpoint so unlucky for me :(
There appears to be two related objects in the response:
"perceived_exertion":1.0,
"prefer_perceived_exertion":false
I guess the second one is something to do with whether the user prefers PE over Heartrate (?)