package Foo::Bar;

my $pkg = __PACKAGE__;

sub new {
  my ($class) = @_;
  my $self = {};

  bless($self, $class);
  $self->{$pkg} = {};

  return $self;
}

sub setvar {
  my $self = shift;
  my $priv = $self->{$pkg};

  $priv->{variable} = 1;
}
[download]

Each method has two variables, $self, which is the blessed hash and is used to call methods, and $priv, which is used to keep the private variables. So, if you use this class, 'variable' will be $self->{Foo::Bar}->{variable}. If a class using the same technique, say, Foo::Bar::Improved also defines the same variable, and inherits from Foo::Bar, then the variable be $self->{Foo::Bar::Improved}->{variable}. No conflict.

You won't see that in the module I posted here though, because it was written a long time ago when I didn't do this yet.