Another way. No special-casing needed for 0, 1 element arrays/lists.

c:\@Work\Perl>perl -wMstrict -le
"my @node_names = qw(PA5098  PA5100  PA5092  PA3175);
 ;;
 for my $i (0 .. $#node_names) {
   my @following = $i+1 .. $#node_names;
   for my $j (@following) {
     print qq{$node_names[$i] $node_names[$j] }, $j - $i;
     }
   }
"
PA5098 PA5100 1
PA5098 PA5092 2
PA5098 PA3175 3
PA5100 PA5092 1
PA5100 PA3175 2
PA5092 PA3175 1
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