...  @array[$n] is a one-element array...

No. As philipbailey has pointed out, it is a (single element) list, and a list is not an array, and an array is not a list! ikegami has many postings on the differences between the two, but I can't come up with a good one at the moment; try Super Search.

Update: For another difference between arrays and lists:

>perl -wMstrict -le
"my @ra = qw(a b c d);
 print qq{@ra};
 for my $elem (@ra) {
   print ++$elem;
   }
 print qq{@ra};
 ;;
 for my $elem (qw(a b c d)) {
   print ++$elem;
   }
"
a b c d
b
c
d
e
b c d e
Modification of a read-only value attempted at -e line 1.
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