package MyTest;
sub new { bless { 'session' => {} }, shift; }
sub param : lvalue {
    my $self = shift;
    my $name = shift;

    if ( ! defined $name && wantarray ) {
        my @out = keys %{$self->{session}};
        return @out;
    }
    $self->{session}->{$name};
}

1;