Sort::Key::Natural seems to have different results based on capitalization. First letter uppercase sorts differently than both all upper and all lower case

   use feature qw( say );
   use Sort::Key::Natural qw( natsort );
   say for natsort qw(     P007b_Yum P007_Yum  );
   say qw(---);
   say for natsort qw(     P007B_YUM P007_YUM  );
   say qw(---);
   say for natsort qw(     P007b_yum P007_yum  );
[download]

Output:

P007_Yum
P007b_Yum
---
P007B_YUM
P007_YUM
---
P007b_yum
P007_yum
[download]

In the first case (uppercase first), the "P007b_" sorts after "P007_". In the other cases (all upper or all lower), it sorts after. As a result, I can't do case-insensitive sorting without changing the results. Can any monks help?

Thanks!