Oops, I forgot what this thread was really about when I posted. $#array is fixed. AvARYLEN isn't changed. AvARYLEN doesn't exist as a field, so don't try to measure its size. The following code shouldn't be executed in 5.10+:

    if (AvARYLEN(thing)) {
      if (check_new(tv, AvARYLEN(thing))) {
        total_size += thing_size(AvARYLEN(thing), tv);
      }
    }
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Test with bleadperl and with those lines present:

perl -MDevel::Peek -MDevel::Size=total_size -wle'
    my @a = qw( a b c );
    Dump(\@a,1);            # Shows no magic
    print total_size \@a;   # 224
    Dump(\@a,1);            # Shows magic

    $#a=$#a;                # Add magic
    Dump(\@a,1);            # Shows magic
    print total_size \@a;   # 224
'
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Test with bleadperl and with those lines commented out:

perl -MDevel::Peek -MDevel::Size=total_size -wle'
    my @a = qw( a b c );
    Dump(\@a,1);            # Shows no magic
    print total_size \@a;   # 168
    Dump(\@a,1);            # Shows no magic

    $#a=$#a;                # Add magic
    Dump(\@a,1);            # Shows magic
    print total_size \@a;   # 224
'
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Update: Fixed reply to reflect the actual usage of AvARYLEN by the module. It's not used to determine the length of arrays as I had originally indicated.

Update: Added tests.