You are getting the real length of the array. Appending additional commas doesn't extend the length of the array. You would have to write this as:

my @array_2 = (1,2,3,4,5,6,7,8,9,undef,undef,undef,undef,undef,undef);
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or

my @array_2 = (1,2,3,4,5,6,7,8,9);
$array_2[14] = undef;
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The reason that trailing commas are not a syntax error is so that you can write code like this:

my %hash = (
    a => 1,
    b => 2,
);
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If the comma added another "undef" to the list then you would get a warning:

Odd number of elements in hash assignment.
Use of uninitialized value in list assignment.
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