on an array it only undefines the value of an element

Actually, it does a bit more than "only" undefining the value. I'm not sure how useful this behavior is, but it's not quite the same as simply setting the value to undef. Here's a code snippet that demonstrates:

$ perl -MData::Dumper -le '
    @array = (1 .. 3);
    delete $array[1];
    print Dumper \@array;
    delete $array[2];
    print Dumper \@array;'
[download]

When run, it outputs the following:

$VAR1 = [
          1,
          undef,
          3
        ];

$VAR1 = [
          1
        ];
[download]

We see that the first delete call does indeed appear to simply set the value to undef, but when we delete again, it demonstrates the difference.

Update: betterworld beat me to it, by a pretty large margin. That'll teach me not to reload before replying. :-)