use strict ; use warnings ;

my $RUNS   = 100_000 ;
my $data   = [[a => 0.1], [b => 0.5], [c => 0.4], [c => 0]] ;

my %count;

foreach (1 .. $RUNS) {
    my $total_weight = $data->[-1]->[1] ;
    if (!$total_weight) {
      $total_weight += $_->[1] for @$data ;
      $data->[-1]->[1] = $total_weight ;
    } ;

    my $pick;
    my $r = rand($total_weight) ;
    foreach (@$data) { if (($r -= $_->[1]) < 0) { $pick = $_->[0] ; last ; } ; } ;
    $count{$pick}++ ;
} ;

while (my ($k, $v) = each %count) {
    printf "%s: %5.2f%%\n", $k, 100 * $v / $RUNS ;
} ;
__END__
c: 39.89%
a:  9.92%
b: 50.19%