use Time::Local;
my $now=time();
my (@t)=localtime($now);

# 8 am today
my $next8am=timelocal(0,0,8,$t[3],$t[4],$t[5]+1900);

# if after 8am today, add one day in seconds.
$next8am+=86400 if ($next8am<$now);

sleep ($next8am-$now);