my @matrix = ( [a => 0.1], [b => 0.5], [c => 0.4] );
my $chosen = pick(@matrix);

sub pick {
    my($choice, $threshold);
    foreach(@_) {
        my $rand = -log(1 - rand)/$_->[1];
        if(not defined $threshold or $rand < $threshold) {
            $choice = $_->[0];
            $threshold = $rand;
        }
    }
    return $choice;
}