$ perl -le 'print ref \substr "foo", 1'
LVALUE

##</code><code>##

$ perl -le 'my $u; sub u :lvalue {$u} print ref \u'
SCALAR

##</code><code>##

#!/usr/bin/perl -l

use strict;
use warnings;

package Append;

sub TIESCALAR { bless \$_[1], $_[0] }
sub FETCH { ${${ $_[0] }} }
sub STORE { ${${ $_[0] }} .= $_[1] }

package main;

sub append :lvalue {
    tie my $v, 'Append', \$_[0];
    $v;
}

my $u='Foo';
(append $u)='Bar';
print $u;

__END__

##</code><code>##

(append $u)='Bar';
print $u;

##</code><code>##

print +(append 'Foo')='Bar';

##</code><code>##

sub append :lvalue {
    local *LVALUE::STORE = sub {
        my $self=shift;
        $$self .= $_[0];
    }
    $_[0];
}