I'd just use what moritz suggested, but the code you posted can be written much more succinctly:

my %group = ( ad1 => 1, ad2 => 2, ad3 => 3 );

my @rndselect = map { ($_) x $group{$_} } keys %group;
$advertinfo->{advert} = $rndselect[rand @rndselect];
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Update: I seem to remember that Knuth had something to say about this in The Art of Computer Programming (probably volume 1). I'll have a look for it tomorrow.