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

{
	package StringThing;
	use Scalar::Util qw/blessed/;
	use Carp;
	use overload fallback => 0,
		'""' => \&str_warn, '.' => \&concat, '.=' => \&concat;
	sub new {
		my ($class, $self) = @_;
		return $self if blessed($self) && $self->isa(__PACKAGE__);
		return bless \"$self", ref($class)||$class;
	}
	sub str_warn {
		carp "unintended stringification";
		goto &str;
	}
	sub str { return ${+shift} }
	sub concat {
		my ($self,$other,$swap) = @_;
		$other=$$other if blessed($other) && $other->isa(__PACKAGE__);
		return $self->new( $swap ? $other.$$self : $$self.$other );
	}
}

my $x = StringThing->new("foo");
my $y = StringThing->new("bar");

my $z = "$x $y";
print $z->str, "\n";

my $zz; $zz = "$x $y";  # warns
print $zz->str, "\n";   # dies

__END__

foo bar
unintended stringification at test.pl line 34.
Can't locate object method "str" via package "foo bar" (perhaps you forgot to load "foo bar"?) at test.pl line 35.