use Time::Local qw( timelocal );

sub get_time_from_local_time_string {
   my ($time_string) = @_;

   my ($M, $D, $Y, $h, $m, $s, $ampm) = 
      $time_string =~ m{(\d+)/(\d+)/(\d+) (\d+):(\d+):(\d+) (\w+)};

   if ($h == 12) {
      $h -= 12 if lc($ampm) eq 'am';
   } else {
      $h += 12 if lc($ampm) eq 'pm';
   }

   return timelocal($s, $m, $h, $D, $M-1, $Y);
}

my $time1 = get_time_from_local_time_string($time_string_1);
my $time2 = get_time_from_local_time_string($time_string_2);

if (abs($time1 - $time2) > 72*60*60) {
   ...
}