fireworks and segfaults

sub put($sky, $row, $column, $char) {
    return $sky unless 0 <= $row <= 24 && 0 <= $column <= 79;
    my $pos = 80 * $row + $column;
    return $sky.substr(0,$pos) ~ $char ~ $sky.substr($pos+1);
}

my $empty_sky = (' ' x 79 ~ "\n") x 24;

my $n = 20;

my @firework_times = map { (0..900).pick }, ^$n;
my @firework_positions = map { (10..69).pick }, ^$n;

my @explosions = map {
    my $p = @firework_positions[$_];
    map { [12, $p, -1 + (rand() - .5) * .7, (rand() - .5) * 2] }, ^10
}, ^$n;

for 0 .. 1000 -> $time {
    my $sky = $empty_sky;

    for @firework_times Z @firework_positions Z @explosions ->
        $t,               $p,                   $e {

        given $time {
            when $t .. $t+12 {
                $sky = put($sky, 24 - ($time-$t), $p, '|');
            }
            when $t+14 .. $t+24 {
                for $e {
                    $sky = put($sky, floor(.[0]), floor(.[1]), '.');
                    .[0] += .[2]; # y movement
                    .[1] += .[3]; # x movement
                    .[2] += .5;   # gravity
                }
            }
        }
    }

    print "\n" x 70;
    print $sky;

    sleep 0.05;
}