Perl code would treat the two the as the same, but user-written XS code makes assumptions.

When I read this, I felt an urge to find Perl code that behaves differently for these arrays :)

Turns out that what this API function is doing would require explicit undefs even if the function was written in Perl. This is a subroutine that modifies its input parameters. In Perl you might write it like this:

use Data::Dumper;

my @a;
@a = (undef) x 5;
my @b;
$#b = 4;

sub modify {
    $_ = 5 for @_;
}

modify(@a);
print Dumper \@a;
modify(@b);
print Dumper \@b;
[download]

Here the subroutine "modify" behaves similar to GetVolumeInformation as it modifies the scalars in the array passed to it.

So the arrays @a and @b should be identical. However, modify() will replace @a with five times 5, but it cannot write to @b:

$VAR1 = [
          5,
          5,
          5,
          5,
          5
        ];
Modification of a read-only value attempted at script.pl line 12.
[download]

Update: Since this seems to depend on the version: I am using Perl 5.20.2 on Debian.