I never noticed before that in order to use the method as an lvalue, you had to use it as an rvalue first to define the function. But you can define AUTOLOAD as an lvalue method, and then it almost works, as the following code demonstrates:

#!/usr/bin/perl

package Foo;
sub new { bless {}, shift }
sub DESTROY {1}
sub AUTOLOAD : lvalue {
  my ( $method ) = $AUTOLOAD =~ /^.*::(.+)$/
    or die "Invalid call to $AUTOLOAD";
  *$AUTOLOAD = sub : lvalue {
    my $self = shift;
    if (@_) {
      $self->{$method} = shift;
      return $self;
    }
    $self->{$method};
  };
  goto &$AUTOLOAD;
  # Uncomment the following 'useless' line
  # and it works:
  # $Foo::baz;
}

package main;
my $foo = Foo->new;
$foo->bar = 4;
print $foo->bar,"\n";
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With the above code you get this error:

Can't modify goto in lvalue subroutine return at ...
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If you uncomment the last line of the AUTOLOAD, you seem to fool the compiler into thinking that AUTOLOAD is going to return a valid lvalue, even though it never actually returns that value, since you goto somewhere else first.