One way to do it yourself would be:

# assuming $_session_time is in seconds.
my $_session_time = $session{time_then};
my $_current_time = time();
my $_time_now = ($_current_time-$_session_time);
my $minutes = int($_time_now / 60);
my $seconds = $_time_now % 60;
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Though this will likely fail during time changes if they are observed where this code is being run.

Alternatives include DateTime, Date::Calc, and Date::Manip.

Code lightly tested. "It works for me."