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);
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