We're told Matthew came home sometime in the AM after a 2-hour round of pickleball, and that he sent his assistant out on errand shortly thereafter. We're told when the assistant returned about 2 hours later, he discovered Matthew unresponsive ... and called 911.

Unclear if what Matthew was doing before his passing played any role in his death at this point -- the investigation into his death is underway.

Law enforcement sources tell us the actor was found Saturday at an L.A.-area home ... where we're told he appears to have drowned. Our sources say first-responders rushed over on a call for cardiac arrest. It's unclear where exactly on the grounds this happened.

Our sources say he was found in a jacuzzi at the home ... and we're told there were no drugs found at the scene. We're also told there is no foul play involved.

Perry is most famous for his role as Chandler Bing on the hit '90s sitcom, which ran for 10 seasons ... and with him in all 234 episodes. His character was a fan fave, as was performance -- mannerisms and lines of which have gone on to be recreated and spoofed by fans all over the world. One comes to mind, in particular ... "Could (blank) BE more .."

While 'Friends' was his biggest claim to fame ... MP had starred/guest starred in countless other TV shows over the years -- such as 'Boys Will Be Boys,' 'Growing Pains,' 'Silver Spoons,' 'Charles in Charge,' 'Sydney,' 'Beverly Hills, 90210,' 'Home Free,' 'Ally McBeal,' 'The West Wing,' 'Scrubs,' 'Studio 60 on the Sunset Strip,' 'Go On,' 'The Odd Couple' and more.