[id://perrin]. Yep clear. Basically, borrowing from Apache::Singleton, here's the relevant code.

package My::Session;

... 

use vars qw( $GLOBAL_SESSION );

...

sub new {
  my( $class, $args ) = @_;
  $args->{log} ||= My::Log->new();
  my $instance = $class->_get_instance( $args );
  unless( defined $instance ) {
    $instance =  $class->_new_instance( $args );
    $class->_set_instance( $instance );
  }

  return $instance;
}

sub _get_instance {
  my( $class, $args ) = @_;
  if( $ENV{MOD_PERL} ) {
    $args->{log}->debug( "Getting mod_perl session instance" );
    my $req = Apache->request();
    my $key = "$class\::_instance";
    return $req->pnotes( $key );
  } else {
    $args->{log}->debug( "Getting cgi session instance" );
    return $GLOBAL_SESSION;
  }
}

sub _new_instance {
  my( $class, $args ) = @_;
  my $self  =  {};
  $self->{log} = $args->{log};
  $self->{log}->debug( "creating new session instance" );
  bless $self, $class;
  # call method to load access privs here
  return $self;
}

sub _set_instance {
  my( $class, $instance ) = @_;
  if( $ENV{MOD_PERL} ) {
    $instance->{log}->debug( "setting new mod_perl instance" );
    my $req = Apache->request();
    my $key = "$class\::_instance";
    $req->pnotes( $key, $instance );
  } else {
    $instance->{log}->debug( "setting new cgi instance" );
    $GLOBAL_SESSION = $instance;
  }
}
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