sub quicksort {
    return unless @_;
    my ($x, @xs) = @_;
    my @pre  = grep{ $_ < $x } @xs;
    my @post = grep{ $_ >= $x } @xs;
    (quicksort(@pre), $x, quicksort(@post));
}

my @a = quicksort(1, 5, 2, 4, 3);
print "@a\n";