You should start by reading the output of "perldoc -f time":

  time    Returns the number of non-leap seconds since what­
          ever time the system considers to be the epoch
          (that's 00:00:00, January 1, 1904 for Mac OS, and
          00:00:00 UTC, January 1, 1970 for most other sys­
          tems).  Suitable for feeding to "gmtime" and
          "localtime".

          For measuring time in better granularity than one
          second, you may use either the Time::HiRes module
          from CPAN, or if you have gettimeofday(2), you may
          be able to use the "syscall" interface of Perl,
          see perlfaq8 for details.
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So, based on this info, you probably want to kill a session when the difference between its start time and the current time is greater than (2*60*60), which is the number of seconds in two hours.