sub list_sort {
    my @vals = @_;

    # trivial case: 0 or 1 items in the list
    return @vals if @vals < 2;

    my $split_val = shift @vals;
    my @smaller = grep { $_ < $split_val } @vals;
    my @larger  = grep { $_ > $split_val } @vals;

    return list_sort(@smaller), list_sort(@larger);
}