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The "Ketamine Queen" -- the last of 5 people charged in Matthew Perry 's death -- is going away to prison, for a long time, after agreeing to plead guilty in the federal case.

Jasveen Sangha , who earned her street handle from slinging the drug in North Hollywood, struck a plea deal on 5 criminal charges -- according to the feds, she will plead guilty to 3 counts of ketamine distribution, one count of distributing ketamine resulting in death and one count of maintaining a drug-involved premises.

In exchange for her guilty plea, the feds are dropping all the other charges for which she'd been indicted. Gallery Launch Gallery Getty

Sangha is still facing some serious time in federal prison -- she could get up to 65 years, but federal prosecutors will recommend

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