At first sight, I spotted a problem right the way. Your code will never work base on your description of the data. Try this (it is your code, with slightly modified testing data):

use Data::Dumper;

my $str="27:43:33:21:23:19:27:6";
my %hash=split/:/,$str;
foreach my $k(keys %hash)
{
    push (@ary, map{$hash{$k}}(1..$k));
}
my $adId=$ary[int(rand(100))];
print Dumper(\%hash);
[download]

Your code does not handle the situation where two ads has the same percentage of show time. Well, the fix is not difficult, so I just leave it to you, just switch key/value, use show number as key.