#! perl -slw
use strict;

package Sensible;

sub TIESCALAR {
	my( $class, $fetch, $store ) = @_;
	no warnings 'redefine';
	*FETCH = *FETCH = $fetch;
	*STORE = *STORE = $store;
	bless [], $class;
}

sub LVALUE (&&) : lvalue {
	my( $fetch, $validate ) = @_;
	tie my $lvalue, 'Sensible', $fetch, $validate;
	$lvalue;
}

sub new {
	my( $class, $init ) = @_;
	return bless \$init, $class;
}

sub attr : lvalue {
	my( $self, $start, $len ) = @_;
	$start ||= 0;
	$len ||= length ${ $_[ 0 ] };
	
	LVALUE { substr( $$self, $start, $len ) } sub{
		warn( 'Bad value' ), return unless $_[ 1 ] =~ m[^[a-z ]+$];
		substr( $$self, $start, $len ) = $_[ 1 ];
	};
}

package main;

my $sensible = Sensible->new( 'The quick brown fox jumps over the lazy dog' );
print $sensible->attr;

$sensible->attr( 10, 5 ) = 'green';
print $sensible->attr;

$sensible->attr( 20 ) = 'did not see the paint tin';
print $sensible->attr;

$sensible->attr( 10, 5 ) = 'ORANGE';
print $sensible->attr;

__END__
[ 2:03:26.03] P:\test>425402-2
The quick brown fox jumps over the lazy dog
The quick green fox jumps over the lazy dog
The quick green fox did not see the paint tin
Bad value at P:\test\425402-2.pl line 31.
The quick green fox did not see the paint tin