I agree with the other posters about using something already built into Windows, but Date::Manip also lets you do this fairly nicely:

use Date::Manip;

my $now = time();
my $next = &UnixDate(&ParseDate("8am"), "%s");  

$next += 24 * 60 * 60 if $next < $now;
sleep($next - $now);
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ParseDate() parses the "8am", and UnixDate() with the "%s" format gives us the number we want. We jump ahead 24 hours if the time parsed is before the current time. A more correct way to do this would be to re-do the ParseDate with "tomorrow at 8am" instead of "8am", since DST could muck this up. I can't find a way to tell ParseDate to use the next future occurrence of "8am" instead of assuming "today at 8am" without messing up the pre-8am case.

ParseDate is also somewhat slow.