use Sort::Key::Natural qw( natkeysort );
my @list = qw(P007b_Yum P007_Yum P007B_YUM P007_YUM P007b_yum P007_yum);
say for natkeysort { lc $_ } @list;

##</code><code>##

$ perl 1202470.pl
P007b_Yum
P007B_YUM
P007b_yum
P007_Yum
P007_YUM
P007_yum