package Object;
use Params::Validate qw(:all);

sub new {
    my $class = shift;
    my %self = validate(@_, {logger => { isa => 'Logger' } } );
    return( bless \%self, $class );
};

# ... more code ...

sub log {
    my ($self, @messages) = @_;
    $self->{logger}->log($self, @messages);
};
[download]

One way to test Object's log method would be to create a mock Logger object with Test::MockObject and pass this to Object->new. Instead, we will make the Test::Class pretend to be a Logger object.

package Object::Test;
use base qw(Test::Class);
use Test::More;

# run $code while pretending that __PACKAGE__ ISA $class
sub _do_as(&$) {
    my ($code, $class) = @_;
    { 
        # we have to make sure the package hash exists
        # for the method dispatch to work
        no strict 'refs'; *{"$class\::"}{HASH} 
    };
    local @Object::Test::ISA = ($class, @Object::Test::ISA);
    $code->();
};

sub setup : Test(setup) {
    my $self = shift;
    _do_as { $self->{object} = Object->new(logger => $self) } 'Logger'
+;
};

sub test_show_answer : Test(3) {
    my $self = shift;
    _do_as { $self->{object}->log("hello", "there") } 'Logger';
};

sub log { 
    my $self = shift;
    is($_[0], $self->{object}, 'passed object to logger');
    is($_[1], "hello", 'passed arg1');
    is($_[2], "there", 'passed arg2');
};
[download]

Sneaky - eh?