Better mousetrap (getting top N values from list X)

[Limbic~Region]

All,
In this node, I pointed out that it is often a waste to sort a list if you were after the top value and that a watermark algorithm should be used instead. I am not sure this rule of thumb holds true when N != 1. The size of list X and N seem to be contributing factors.

Here is the code I used in that node:

sub top_x {
    my ($list, $x) = @_;
    $x--;
    my @top;
    $#top = $x;
    for my $item ( @$list ) {
        next if defined $top[ -1 ] && $item <= $top[ -1 ];
        for my $id ( 0 .. $#top ) {
            $top[ $id ] = $item and last if ! defined $top[ $id ];
            if ( $item > $top[ $id ] ) {
                @top[ $id .. $#top ] = ($item, @top[ $id .. $#top - 1]
+);
                #splice(@top, $id, 0, $item), pop @top;
                last;
            }
        }
    }
    return @top;
}

After writing it, I wondered:

• At what value of N does one of the 2 methods (splice vs array slice) beat the other?
• Does the length of the list X and the value of N have to be considered together when deciding?
• Under what circumstances is sorting better than watermarks?
• Is there a better way to keep track of the watermarks other than the one I have shown?
• Is there a better method, other than sorting and watermarks, for determining the top N of a list?