I do this by subclassing Test::MockObject...

package Test::AutoMock;
use base 'Test::MockObject';

# This is a stripped-down version of some of my own testing
# code, so this is untested, but should work...
sub dispatch_mocked_method {
    my $self = $_[0];
    my $sub = splice( @_, 1, 1 );
    my $subs = Test::MockObject::_subs( $self );
    $self->log_call( $sub, @_ );

    if ( exists $subs->{ $sub } ) {
        goto &{ $subs->{ $sub } };
    } else {
        return 1;
    }
}
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dispatch_mocked_method is called by Test::MockObject::AUTOLOAD, and carps when you call a method that hasn't been mocked, so if you just replace the carp with a return 1; you get the 'return true unless overridden' behaviour. I also moved the log call outside the if, so that the call log can include both mocked and default method calls.