#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my %hoh = (
        elt1 => {
                A      => "ccc",
                B       => "ccc",
                c       => "ccc",
        },
        elt2     => {
                A      => "ccc",
                C      => "ccc",
                D      => "ccc",
                B       => "ccc",
        },
        elt3    => {
                A      => "ccc",
                E      => "ccc",
                C      => "ccc",
        },
 );

my @keys=keys %hoh;
my %saw;
for (@keys) {
    $saw{lc,}++ for keys %{ $hoh{$_} };
}

for (@keys) {
    my $h=$hoh{$_};
    $saw{lc,}<@keys and delete $h->{$_} for keys %$h;
}

print Dumper \%hoh;

__END__
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