use strict;
use warnings;
use feature 'say';

my @list = qw(P007b_Yum P007_Yum P007B_YUM P007_YUM P007b_yum P007_yum);

my @sorted = sort { lc $a cmp lc $b } sort { $b cmp $a } @list;
say for @sorted;

##</code><code>##

$ perl sort_lc.pl
P007_yum
P007_Yum
P007_YUM
P007b_yum
P007b_Yum
P007B_YUM