#!/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__