Note that (internal, C-level) arrays contain pointers to SVs.

It could mark a range as empty without actually filling in each bucket.

I just tried it. Setting element 10000 of an array takes 0.35 MB more RAM than setting element 0 on perl 5.8.8 for OS X on Intel. So, there's a cost, although not a large one.

It could mark a range as empty without actually filling in each bucket.

Yes, and then you'd need a data structure to keep track of which ranges are empty. Wait, I got it! We'd use a hash to keep track which elements in the array are 'valid'! But then, you might as well use a hash to begin with.

Furthermore, it doesn't save any memory. Even if you don't initialize in the first 1000 elements if you do:

    my @array;
    $array[1000] = 1;
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you still need to allocate space for 1001 pointers. Initialized or not.

I just tried it. Setting element 10000 of an array takes 0.35 MB more RAM than setting element 0 on perl 5.8.8 for OS X on Intel. So, there's a cost, although not a large one.

That's about 36 bytes/element. More that I expected. An undefined value takes 20 bytes in Perl. A pointer takes 4 bytes (32-bit platform). So, even if undefined values aren't shared, I'd expect less.

However, I cannot recreate this:

$ perl -MDevel::Size=:all -wE '$a[1] = 1; say total_size \@a'
148
$ perl -MDevel::Size=:all -wE '$a[10000] = 1; say total_size
\@a'
40136
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which is a difference of about 40000 bytes, aka 10000 pointers. Replacing Devel::total_size with <c>`grep VmRSS /proc/$$/status` confirms the difference of 40000 bytes.