You haven't stored anything in the hash yet, so $_[0]->{val}->{one} returns undef, which isn't an lvalue.

Erm ...

{
    package foo;
    use Tie::IxHash;
    tie(my %h, 'Tie::IxHash', qw(one two three four));

    sub new { bless { val => \%h } }
    sub val : lvalue  { $_[0]->{val}->{one} }
}
use Data::Dumper;
my $obj =  foo->new;
print Dumper( $obj );

__output__

$VAR1 = bless( {
                 'val' => {
                            'one' => 'two',
                            'three' => 'four'
                          }
               }, 'foo' );
[download]

I figured that the issue might be in the FETCH method of Tie::IxHash, but I thought that tied variables were meant to behave just like normal variables (in terms of interface at least), apparently not.

_________
broquaint