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 ($key))} || 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 code 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