If you produce a copy constructor by overloading '=', I believe the reflexive mutators will be autogenerated to do the right thing. There is explanation on pp 7-8,14 of perldoc overload.

I found much of that goop called documentation unintelligible and have implemented methods for -, -=, and =. I only see this problem in -=. The portions covering this part of overload just weren't clear on what they meant. Also, while there were some general warnings about taking references to $_[0] there was nothing about the contents of data structures being magically altered when references to $_[0] were present.

I think if i understand this properly you can see the same thing at work without overload. Try this:

package T;
use Data::Dump::Streamer;

sub foo { \$_[0] }
sub bar { my $x=shift; \$x }

my $o=bless [],'T';

Dump($o->foo,$o->bar,$o)->Names(qw(foo bar o))->Out();
__END__
$foo = \$o;
$bar = \do { my $v = 'V: $o' };
$o = bless( [], 'T' );
$$bar = $o;
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What happening here is that you seeing the difference between copy semantics (what happens with assignment) and aliasing semantics (what happens when you take a reference to an alias).

But i know you well enough to know this isnt a surprise to you, so Im wondering if you could explain in more detail perhaps with a minimal case we can run to see the effect you mean.

Here's my short test code which demonstrates the data structure prior to returning and how it instamagically turned into a self-ref afterward.

Here's the data before returning and after returning it from the -= method.

$expected = bless( \bless( \1, 'T' ), 'T' );
$got = bless( \$got, 'T' );
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And here's the sample code.

package T;
use Data::Dump::Streamer;
sub D {
    my ( $name, $val ) = @_;
    Dump()
      ->Purity( 0 )
        ->Names( $name )
          ->Data( $val )
            ->Out();
    return;
}

use overload(
    '-=' => sub {
        my $new = bless \ $_[0], "T";
        D( new => $new );
        return $new;
    },

    '""' => sub {
        my $self = shift;
        D( self => $self );
        return $$self
    }
);

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

package main;
$o = T->new( 1 );
$o -= 2;
"$o";
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There are occasionally times when you wish to be extra careful to be using an alias to the same variable your caller is using. I was doing lots of that so it was normal and expected for me to be working on things in @_ directly. That isn't the case for most code.

It's a very wide spread myth that objects are blessed references. In fact, they are references to blessed variables. The difference? The thing that is blessed.

I agree that the syntax of bless is misleading. But if you bless \$foo, it is $foo that is actually blessed. This means that you can choose to return \$foo; and it'll still work, even though the scalar assigned for the second reference you're creating is an entirely different one. It's not the reference, but $foo that is blessed.

@_ is an array of aliases. These are aliases for the variables used in the call. When such a value/variable is mutable, the element in @_ is automatically as mutable, because it's the same thing. This is a danger with using any element of @_ directly, not just when they're overloaded, and not just with bless. Even assignment to one of the elements is dangerous. The lesson is simple: if you want to change something without changing it, copy and change the copy, so the original stays intact.