Doesn't really address your overall efficiency question, but I see over and over people forgetting (or never having known) that push takes a LIST, not just a single scalar:

        while ( $group->{$advert} > 0 ) {
            push( @rndselect, $advert );
            $group->{$advert}--;
        }#while
[download]

becomes

        push( @rndselect, ($advert) x $group->{$advert} );
[download]