use strict;
use warnings;
use Data::Dump qw/pp dd/;


my @AoH =
  (
   {AB => 100, NN => 200, XYZ => 400},
   {AB => 100, XYZ => 400, MM => 300},
   { map { ("A$_" => $_) } "A".."C" }
  );

my %H;
%H = (%H,%$_) for @AoH;
dd keys %H;

my %count;
map { map { $count{$_}++ } keys %$_ } @AoH;
dd sort { $count{$b} <=> $count{$a} || $a cmp $b } keys %count;

##</code><code>##

("XYZ", "NN", "MM", "AC", "AA", "AB")
("AB", "XYZ", "AA", "AC", "MM", "NN")