very interesting question and discussion. I am not able to offer a solution but this lead to me to something different: temporary hide or mask the value of some key of a tied hash.

Mostly copying ikegami's SO answer I got something working.

I'm sure some monk can play also with FIRTSKEY and NEXTKEY vodoo to make some key temporary invisible (see the comment line in fetch..), not me :)

package MaskKeys;

use strict;
use warnings;
use Tie::Hash ();
our @ISA = 'Tie::ExtraHash';

sub TIEHASH {
        my $class = shift;
        my $content = shift;
        my $tomask = shift;
        my $self = bless([{@$content}, {}]);
        $self->[1]{$_} = 1 for @$tomask;
        return $self;
} 
sub FETCH { 
        my $self = shift;
        my $key = shift;
        if ( $self->[1]{$key} ){
            return 'NA';
            # {return $self->{ $self->FETCH($self->SUPER::NEXTKEY ($ke
+y))} || undef ;
        }
        else { return $self->[0]{$key} }
}
sub mask_key{
    my $self = shift;
    my $key = shift;
    $self->[1]{$key} = 1;    
}
sub unmask_key{
    my $self = shift;
    my $key = shift;
    # delete @{ $self->[1] }{ $key }; unecessary from cut and paste co
+de that accepted a list
    delete $self->[1]{$key};    
}

package main;
                         # content                       # to mask
tie my %hash, 'MaskKeys',[schema=>11111, aaaaaa=>33333], ['schema'];

print "\noriginally masked: \n";
print "$_ -> $hash{$_}\n" for keys %hash;

print "\nunmasked: \n";
tied(%hash)->unmask_key('schema');
print "$_ -> $hash{$_}\n" for keys %hash;

print "\nmasked again: \n";
tied(%hash)->mask_key('schema');
print "$_ -> $hash{$_}\n" for keys %hash;


__END__

originally masked:
aaaaaa -> 33333
schema -> NA

unmasked:
aaaaaa -> 33333
schema -> 11111

masked again:
aaaaaa -> 33333
schema -> NA
[download]

L*