This is a variant on duff's method that's a bit faster, and I have here performance numbers to demonstrate it: 
my @a = split/:/,$str;
@a % 2 && die;                # not an even number of items
my $r = int(rand(100));
my ($adId, $p);
while (($p,$adId) = splice @a,0,2) {
   if ($r < $p) {last;}
   $r -= $p;
}

[download]
Now, the performance numbers; it's pretty easy to test all the variants given so far with Benchmark.pm: 
            Rate origCode     duff       QM      L~R      ccn   fizbin
origCode  4029/s       --     -34%     -77%     -82%     -87%     -89%
duff      6137/s      52%       --     -65%     -73%     -81%     -83%
QM       17360/s     331%     183%       --     -24%     -46%     -53%
L~R      22908/s     469%     273%      32%       --     -29%     -38%
ccn      32106/s     697%     423%      85%      40%       --     -12%
fizbin   36690/s     811%     498%     111%      60%      14%       --

[download]
I have the benchmark code, but it's basically just a bunch of cuts and pastes of the code that's been posted on this thread, so I'll only post it if asked.

Update: Fixed oops in code, updated performance figures. 

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/

[download]

In reply to Re: $str to %hash to @ary by fizbin
in thread $str to %hash to @ary by abaxaba

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