The best way I can think of (as in, requiring the least amount of existing codebase changes) is to create a "proxy object". In other words, tie the original hash to your own class which looks for the key you want to have tied separately and handles it specially.

#!/usr/bin/perl -l

use warnings;

package Tie::MyHash;

use Tie::Hash;
@ISA = ('Tie::StdHash');

use Tie::IxHash;
use Tie::Scalar::Timeout;

sub TIEHASH {
  my $class = shift;

  my $hash  = {};

  tie %$hash, 'Tie::IxHash';
  tie ${$hash->{'special'}},     'Tie::Scalar::Timeout', EXPIRES => '+
+2s';

  return bless $hash, $class;
}

sub FETCH {
  my ($this, $key) = @_;

  if($key eq 'special') {
    print tied(${$this->{'special'}}) if $key eq 'special';
    return ${$this->{'special'}};
  } else {
    print tied(%$this);
    return $this->{$key};
  }
}

sub STORE {
  my ($this, $key, $value) = @_;

  return ${$this->{'special'}} = $value if($key eq 'special');
  return $this->{$key}         = $value;
}

sub AUTOLOAD {
  my $this = shift;
  my $sub  = $AUTOLOAD;
  $sub =~ s/.*:://;

  tied(%$this)->$sub(@_);
}

# Tie::IxHash doesn't have a DESTROY method, so we need one
sub DESTROY {}




package main;

tie my %hash, 'Tie::MyHash';

$hash{'mundane'} = 'foo';
$hash{'special'} = 'bar';

print $hash{'mundane'};
print $hash{'special'};

print "Keys: ", join(", ", tied(%hash)->Keys(0, 1));

sleep 3;
print $hash{'special'} || 'undef';
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Update: Hrm. I just realized that the "special" key will not be considered part of the Tie::IxHash at all, for ordering purposes and such, so this isn't really a good solution either. I'll have to think about that...

Update2: It was straight-forward to change it so that the "special" key was stored in the Tie::IxHash. Updated the code accordingly.

bbfu
Black flowers blossum
Fearless on my breath