#!/usr/bin/perl 

use warnings;
use strict;

use Data::Dumper;

my @original_array = qw( THIS This That That ThAT ThAt ThAt THIS These Those );
my %results;

map { $results{uc($_)}->{$_} = undef } @original_array;

print "Original\n", Dumper(@original_array), "\n";
print "Result\n", Dumper( %results ), "\n";

__END__
Original
$VAR1 = 'THIS';
$VAR2 = 'This';
$VAR3 = 'That';
$VAR4 = 'That';
$VAR5 = 'ThAT';
$VAR6 = 'ThAt';
$VAR7 = 'ThAt';
$VAR8 = 'THIS';
$VAR9 = 'These';
$VAR10 = 'Those';

Result
$VAR1 = 'THAT';
$VAR2 = {
          'ThAT' => undef,
          'ThAt' => undef,
          'That' => undef
        };
$VAR3 = 'THIS';
$VAR4 = {
          'THIS' => undef,
          'This' => undef
        };
$VAR5 = 'THOSE';
$VAR6 = {
          'Those' => undef
        };
$VAR7 = 'THESE';
$VAR8 = {
          'These' => undef
        };