foreach my $i ( 0 .. 1520) { 

    foreach my $j ( $i + 1 .. 1520 ) {

        my $dist = sqrt(
                ($arrayx[$i] - $arrayx[$j])**2 +
                ($arrayy[$i] - $arrayy[$j])**2 +
                ($arrayz[$i] - $arrayz[$j])**2
            );

        ## Adding $i and $j to your output will let you know what that

        print "$i <> $j : $dist\n";

    }
	}
exit 0;