I did it for MLDBM (here: MLDBM::Easy), so I'll do it for Tie::IxHash. This simple module will be on CPAN in an hour or two as Tie::IxHash::Easy (if you have a better name, /msg me or reply here).

package Tie::IxHash::Easy;
use Tie::IxHash;

@ISA = 'Tie::IxHash';

sub STORE {
  my ($self, $key, $value) = @_;
  tie %$value, 'Tie::IxHash::Easy' if ref($value) eq "HASH";
  $self->SUPER::STORE($key, $value);
}

1;
[download]

The module will have strictures and warnings for the fun of it, and documentation, and a version number, of course.

To use it, just use Tie::IxHash::Easy instead of Tie::IxHash in your code. You only need to tie the outermost hash. Hashes created inside the tied hash will be auto-tied by Tie::IxHash::Easy::STORE.