During the U.S. Attorney's news conference Thursday, prosecutors claimed Dr. Salvador Plasencia came to the actor's residence a little over 2 weeks before his death on Oct. 28, 2023, and administered a rather large dose of ketamine via injection.

Prosecutors say the injection caused Perry's blood pressure to spike and made him freeze up ... which apparently freaked out Plasencia so much he told Perry's assistant, Kenneth Iwamasa, they should not administer a dose that large again.

But, prosecutors claim the 2 men didn't stop injecting Perry altogether -- they assert Iwamasa injected his boss close to 30 times in the days leading up to his death, and Plasencia even met Perry in a Long Beach parking lot to administer ketamine.

And, now details are emerging about the last hours of Matthew's life ... where he allegedly had Iwamasa shoot him up three times with Ketamnine -- including the last time when he apparently asked Kenneth to "shoot me up with a big one."

Both doctors were charged yesterday in connection to his death, along with Iwamasa, an alleged drug broker named Erik Fleming and Jasveen Sangha ... the woman who's been dubbed the "Ketamine Queen of Los Angeles."

As we've reported, Plasencia allegedly expressed a desire to upcharge Perry a ton for the drug, texting his other defendants, "I wonder how much this moron will pay" ... in reference to Matthew.