%hash = split /X/, 'fooXbar' # added the missing comma ;)

##</code><code>##

#!/usr/bin/perl
use strict;
use Data::Dumper;

tie my %hash, 'ExpandingHash';

$hash{'still'} = 'here';
%hash = split /X/, 'fooXbar';
print Dumper(\%hash);

# I also added a new clear method, which you could use like this.
# tied(%hash)->FORCE_CLEAR;


__OUTPUT__
$VAR1 = {
          'foo' => 'bar',
          'still' => 'here'
        };

#==================================================================
package ExpandingHash;
use strict;

sub TIEHASH  { my %self; bless \%self, shift }
sub FETCH    { $_[0]->{$_[1]} }
sub STORE    { $_[0]->{$_[1]} = $_[2] }

sub FIRSTKEY { each %{$_[0]} }
sub NEXTKEY  { each %{$_[0]} }

sub CLEAR    { } # NOT IMPLEMENTED
sub DELETE   { delete $_[0]->{$_[1]} }
sub EXISTS   { exists $_[0]->{$_[1]} }

sub FORCE_CLEAR { %{$_[0]} = () }