#!/usr/bin/perl
use strict;
use List::MoreUtils qw(uniq);
use Data::Dump 'pp';

my %files = (
  'ABC' => {
    'file1' => 6,'file2' => 5,'file3' => 8, 'file4' => 4,    
  },
  'XYZ' => {
    'file2' => 5,'file5' => 6,'file8' => 8, 
  },
);

my @files = uniq map {keys %{$files{$_}}} keys %files;
pp \@files;