my $RUNS = 100_000;
my $data = [[a => 0.1], [b => 0.5], [c => 0.4]];

my %count;

foreach (1 .. $RUNS) {
    my $pick;
    my $w = 0;
    rand($w += $$_[1]) < $$_[1] and $pick = $$_[0] for @$data;
    $count{$pick}++;
}
while (my ($k, $v) = each %count) {
    printf "%s: %5.2f%%\n", $k, 100 * $v / $RUNS;
}
__END__
c: 39.88%
a:  9.93%
b: 50.19%