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my $a_ref = [ qw/ A B C D E F G H / ];
my $b_ref = [ qw/ 7 6 5 4 3 2 1 0 / ];

my @KEY = map { $a_ref->[$_], $b_ref->[$_] } 0 .. $#{$a_ref};

use List::MoreUtils qw( zip );

...
my $b_ref = [ qw/ 7 6 5 4 3 2 1 0 / ];

my @KEY = zip @{$a_ref}, @{$b_ref};