If the dates are in epoch seconds, take the difference and divide it by the number of seconds in a day (which is 86400). Like so:

my $days_difference = int(($time1 - $time2) / 86400);
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The int() there, by the way, chops off the decimal so you don't end up with something like 4.3231235. If you're curious, the remainder of that division is the number of seconds left after finding the number of days, so if you want "x days, x hours", you can do this next:

my $hours_left = int((($time1 - $time2) % 86400) / 3600);
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That should give you an idea. If your dates aren't in epoch seconds, you'll either have to get them there somehow, or check out one of the Date::* modules.