I seem to be having a problem STOREing multidimensional values in a TIEd hash. I'm now going round in circles because even my minimal test case (which is typically all I need to produce to fix my own problem) isn't doing what I expect.

I've reviewed How to tie multilevel (multidimensional) hash? and the Tie::Hash documentation, and I've frankly run out of brain power.
I'm ultimately trying to concatenate keys of the subhashes together where the top-level key matches ignoring case. However, the code below doesn't even warn twice (it warns once with an empty hashref), suggesting it's broken in a stupid way I can't spot, or I'm not grokking Ties.

use warnings;
use strict;
use Data::Dumper;
use Tie::Hash;

package FlatHash;
use Data::Dumper;
our @ISA = qw(Tie::ExtraHash);

sub STORE {
    my $me    = $_[0];
    my $key   = $_[1];
    my $value = $_[2];
    $key = lc $key;
    $_[0][0]{$key} = $value;

    if(ref $value eq "HASH") {
        warn Dumper($value);
    }
    else {
        print "This isn't a hash\n";
    }
}

package main;

tie my %flat, "FlatHash";
$flat{Monkey}{habits} = "Eating";
$flat{monkey}{habits} = "Scratching";

warn Dumper(\%flat);
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Ultimately I'd like the final datastructure to be something like:

$VAR1 = {
          'monkey' => {
                        'habits' => ['Eating', 'Scratching']
                      }
        };
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Any hints gratefully received.