#!/usr/bin/perl
use strict;
use warnings;

package Tie::EnsureLowercaseSubstr;
use Carp;

sub TIESCALAR {
	my $class = shift;
	my %self;
	@self{ qw( strref offs len ) } = @_;
	bless \%self, $class;
}

sub STORE {
	my $self = shift;
	my ( $value ) = @_;
	croak 'Bad value' unless $value =~ /^[a-z]+$/;
	substr( ${ $self->{ strref } }, $self->{ offs }, $self->{ len }, $value );
}

sub FETCH {
	my $self = shift;
	substr( ${ $self->{ strref } }, $self->{ offs }, $self->{ len } );
}

package SillyClass;

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

sub modifySubstring: lvalue {
	my( $self, $start, $length ) = @_;
	tie my $response, 'Tie::EnsureLowercaseSubstr', \$$self, $start, $length;
	$response;
}

package main;

my $silly = SillyClass->new( 'a teststring' );
$silly->modifySubstring( 3, 3 ) = 'ABC';