According to the perldoc for time:

       time    Returns the number of non-leap seconds since whatever t
+ime the
               system considers to be the epoch (that's 00:00:00, Janu
+ary 1,
               1904 for Mac OS, and 00:00:00 UTC, January 1, 1970 for 
+most
               other systems).  Suitable for feeding to "gmtime" and "
+local-
               time".
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So it is in seconds. To find the difference between your timestamp and the current time, you just subtract.

if ( (time() - $timestamp) > (60 * 60 * 2) ) {
    # Do what you need to expire the session
}
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