xdg,
I am afraid I have no idea how either of the modules you suggested would come close to accomplishing what davis is wanting to do.

tie my %flat, "FlatHash";
$flat{Monkey}{habits} = "Eating";
$flat{monkey}{habits} = "Scratching";
warn Dumper(\%flat);
__END__
Ultimately I'd like the final datastructure to be something like:

$VAR1 = { 'monkey' => {
    'habits' => ['Eating', 'Scratching']
    }
};
[download]

To me it sounds like a variation on Data::MultiValuedHash or less likely Hash::MultiKey except that the top level is also case insensitive.