use strict;

my %HASH;

use Tie::CaseInsensitive;
tie %HASH, 'Tie::CaseInsensitive';

$HASH{BoB}  = 1; 
$HASH{bob}  = 2;
$HASH{BOb}  = 3;

tie %{$HASH{bill}}, 'Tie::CaseInsensitive';
$HASH{bill}{jones} = 3;
$HASH{BILL}{JONES} = 2;

use Data::Dumper;
print Dumper \%HASH;