my $group = { ad1 => 5, ad2 => 10, ad3 => 20 };

    my $weighttotal  = 0;
    my @rndselect = ();
    foreach my $advert ( keys %$group ) {
        $weighttotal += $group->{$advert};
        while ( $group->{$advert} > 0 ) {
            push( @rndselect, $advert );
            $group->{$advert}--;
        }#while
    }#loop
    my $rannum = rand($weighttotal);
    $rannum  = int($rannum);
    $advertinfo->{advert} = $rndselect[$rannum];