Two things I should point out: first, your lead comment suggests that the routine be called with a reference to @Matrix, but the routine itself never uses any arguments -- instead it uses the global (or perhaps the file scoped lexical) array of the same name.

More problematic than that is that you've used the ^ where you really wanted to use ** operator (assuming your distance is meant to be standard Euclidean distance: square root of sum of squared differences). Here's a quick rewrite (which also does not require some arbitrary large number to initialize the nearest neighbor distance):

sub nneighbours {
    die "Bad Input Matrix: must be even" if @_ % 2; 
    my @mat = @_;
    my $sum = 0;
    my $limit = $#mat;
    for(my $i = 0; $i < $limit; $i += 2){
        my $nearest;
        for(my $j = 0; $j < $limit; $j += 2){
            next if $i == $j;
            my $dist = sqrt(($mat[$i] - $mat[$j]) ** 2 +
                            ($mat[$i+1] - $mat[$j+1]) ** 2);
            $nearest = defined $nearest?
                       ($nearest > $dist ? $dist : $nearest)
                       : $dist;
        }
        $sum += $nearest;
    }
    return $sum;
}
[download]

I thought I had understood your comment, but there is still something I can't figure out. Why does:

my @test=qw(1 2 3 4 5 6);
(@test % 2) ? print "uneven\n" : print "even\n";
print "end\n";
[download]

Work fine, while the two versions below result in "uneven" in the first case and "even" in the second, when both should be reported as "even".

@test=qw(1 2 3 4 5 6);
testthearray1(\@test);
sub testthearray1{
(@_ % 2) ? print "uneven\n" : print "even\n";
}
testthearray2();
sub testthearray2{
(@_ % 2) ? print "uneven\n" : print "even\n";
}
[download]

The BlueCamel says "Any arguments passed to a Perl routine come in as the array @_" (pg 219), and this is clearly what you assume happens when you assign my @mat = @_ in the second line. When I include something like  print "@_" to the second line in the  sub testhearray (1 and 2), it shows up as an undefined array (or rather, ARRAY(0x9b25fd8) in the first, and nothing in the second).

I also find it strange that it is the first of the two subs that apparently requires a reference to @test instead of @_, when this is the one which includes a reference to the array when calling the subroutine.

What am I overlooking? (this humbled novice asks).

Elias if you pass a reference to an array then @_ receives that reference -- that is, one element (no matter how big the array is). In testthearray1(\@test) you are only passing in one element. In testthearray2() you aren't passing in anything at all.

The routine I showed should just be called as:

   my @array = (1,2,3,4,5,6);
   my $ndist_sum = nneighbours(@array);
[download]

@test=qw(1 2 3 4 5 6); 
testthearray1(\@test); 

sub testthearray1{ 
  (@_ % 2) ? print "uneven\n" : print "even\n"; 
}
[download]

You're passing one thing, all the time. That's always odd (and odd!). Remove the \ in front of @test.

-- Randal L. Schwartz, Perl hacker