Katy Perry could really hear her fans 'Roar' last night ... probably screaming out in horrible pain -- 'cause one of them touched a transformer and ended up on fire, and nearly disfigured the pop star's face with sparks.

The singer-songwriter was signing autographs outside "Jimmy Kimmel Live!" Wednesday night ... and while making her way down the fence to interact with her fans, sparks exploded in her direction just a couple feet away, causing Katy to duck.

It turns out one of Perry's fans accidentally touched a transformer -- causing a surge of electricity and sparks to spew out ... setting the KP fan's coat on fire.

You gotta see the pics ... a small fire burns on the woman's elbow -- and, while she's finally able to put it out, it's still smoldering at the end.

Fortunately, no one was hurt too badly ... and, Katy even went back to signing autographs later in the evening -- though she did so from the safety of her car the second time around.

Perry -- who will perform at L.A. FireAid, the relief concert for the devastating wildfires that burned through huge areas of L.A. earlier this month -- stopped by Jimmy Kimmel's show to talk about her new "The Lifetimes" tour, kicking off April 23 in Mexico City.

The new tour is sure to get fans hot under the collar ... or on the elbow, it looks like!