Sia's talents by now are well-known, both as a songwriter and pop singer herself, and is credited as one of the writers on "Perfume". While an acoustic version of the track featuring Sia can be found around the web, only non-official demo versions exist online of Sia's take. Still, compare the two singers and it's pretty clear who's voice that is in the video of Spears' performance:
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If you listen closely to the performance video, you can make out some of Spears' vocals under Sia's more charactered voice. That's because, most likely, Spears uses Sia's track as a in-monitor guide when she sings. Only this time it was put into the house mix, making for an embarrassing accident.