The statement from the young man — whose name is protected by a publication ban — was made in an email to the coroner's inquest in Saint John, N.B., into Perry's death.
Perry, 22, was unresponsive when she was found on the floor of the Saint John Regional Hospital on Feb. 14, 2012 with a blue hospital house coat wrapped around her neck.
The inquest heard earlier that the young man had told nurses he had been with Perry in the amphitheatre but left because she was acting strange.
Police considered the young man a suspect when he was questioned the day after Perry's death, but he was released and no charges were laid.
Pathologists were not able to confirm the cause of death, but the probable cause was deemed to be asphyxia as a result of neck compression.
The young man was issued a summons to appear at the inquest on Monday but he declined because of his mental health issues. He is in another province, and the coroner doesn't have the power to force him to comply.
However, lead investigator Const. Stephen Davidson read a statement the young man sent to the inquest. In it he says he was with Perry in the amphitheatre but he left after she was talking about an alien that was following her. He said that Perry said "Tonight's the night. It's coming tonight."
He wrote that as he was leaving, Perry held onto the sleeve of his house coat and he slipped out of it, leaving it with her.
Davidson told the court that DNA samples were taken and while the young man's DNA was found on the house coat, it was not found anywhere on Perry.
He also said he and other police officers tried tying a similar hospital house coat around their own necks. He said while they were able to get it tight, they could not get it tight enough to choke themselves or even pass out.
Later in the day, Susan Haley, the director of addiction and mental health services for the Horizon Health district told the inquest that a number of changes have been made at the hospital as a result of a review done after Perry's death.
Police and the coroner were not called right away after Perry's death in 2012. The hospital did call police to ask for help notifying next of kin.
Haley says the policy at the time was to notify police of any event that is criminal in nature.
Since March 2014 a new policy says police are to be notified of any misadventure, suspicious death or any event that is of criminal nature.
The manager of security must now manage control of the scene until police arrive. In Perry's case, the body and any medical debris used during her resuscitation effort were removed from the amphitheatre before police arrived.
Haley says the hospital now has more security cameras.
She read a list of eight recommendations submitted by Horizon Health to the inquest.
Among them is to implement community treatment orders in New Brunswick. These would require a psychiatric patient, who is not in hospital, to take their medications and undergo treatment. New Brunswick and the Northwest Territories are the only places in the country without community treatment orders.
Perry's sister, Tasha King, told the inquest that Serena had her problems but just wanted to be able to have her own place to live and be as normal as possible.
King said she believes a community treatment order would have made a difference for her sister.
The inquest is expected to go to the five-member jury on Tuesday. They are to decide if the cause of Perry's death was suicide, homicide, an accident, or undetermined. They will also make recommendations on how to prevent a similar death in the future.