I feel silly having to demonstrate such trivial solutions to someone who otherwise seems quite knowledgable, but here goes:

#!/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';
[download]

Part two:

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

package Tie::EvenUniqueTrueArray;
use Tie::Array;
use base qw( Tie::StdArray );
use Carp;

my( %Array, %Start, %End );

sub TIEARRAY {
    my $class = shift;
    my ( $array, $start, $end ) = @_;
    my $self =  [ @{ $array }[ $start .. $end ] ];
    ( $Array{ 0 + $self }, $Start{ 0 + $self }, $End{ 0 + $self } ) = 
+( $array, $start, $end );
    bless $self, $class;
}

sub STORE {
    my $self = shift;
    my ( $idx, $value ) = @_;
    $self->[ $idx ] = $value;
    my %seen;
    croak 'Bad value'
        if !$value or ( $value & 1 ) or grep $seen{ $_ }++, @$self;
}

sub DESTROY {
    my $self = shift;
    @{ $Array{ 0 + $self } }[ $Start{ 0 + $self } .. $End{ 0 + $self }
+ ] = @$self;
    delete $_->{ 0 + $self } for \( %Array, %Start, %End );
}

package SillyClass2;

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

sub modifySubset: lvalue {
    my( $self, $start, $end ) = @_;
    tie my @response, 'Tie::EvenUniqueTrueArray', \@$self, $start, $en
+d;
    @response;
}

package main;

my $silly = SillyClass2->new( map $_ * 2, 1 .. 50 );
( $silly->modifySubset( 15, 16 ) ) = ( 20, 22 );
( $silly->modifySubset( 15, 35 ) ) = ( 1 .. 20 );
[download]

All of that was all there in my previous reply.

Goes to show why it's nice that Perl6 will only require a fraction of the red tape, though.

Makeshifts last the longest.

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