package Defined;

use strict;
use warnings;

sub TIESCALAR { 
	my $class = shift;
	my $name = shift;
	my $self = { 'NAME' => $name,
		     'DEFINED' => undef, };
	return bless ($self, $class);
}

sub FETCH {
	my $self = shift; 
	my $name = $self->{'NAME'}; 
	my @caller = caller; 
	if (defined $self->{'DEFINED'}) {
	       print STDERR "@caller -> $name defined\n";
        } else {
 		print STDERR "@caller -> $name undefined\n";
	}		
}

sub STORE {
	my $self = shift;
	my $value = shift;
	my $name = $self->{'NAME'};
	my @caller = caller;
	$self->{'DEFINED'} = $value;
	print STDERR "@caller -> $name defined with value $value\n";
}

1;

##</code><code>##

#!/usr/bin/perl

use Defined;

use strict;
use warnings;

my $var;
tie $var, 'Defined', '$var';

print $var;
$var = 1;
print $var;
$var = 5

##</code><code>##

bash-3.00$ ./test.pl
main ./test.pl 11 -> $var undefined
main ./test.pl 12 -> $var defined with value 1
main ./test.pl 13 -> $var defined
main ./test.pl 14 -> $var defined with value 5