Magento – How to print or debug array or object in Magento 2

Mage::log(print_r($arr, 1), null, 'logfile.log');

Adding a second parameter to print_r will return a string with printed variable.
[EDIT]
based on the comments below I feel obligated to offer other options for logging an array.

Mage::log($arr, null, 'logfile.log');

or if you need a string prefix to the array

Mage::log('prefix'.Zend_Debug::dump($arr, null, false), null, 'logfile.log');

The second parameter of Zend_Debug::dump() is a label. If this is not null it will be added to before the array dump.
The third parameter of Zend_Debug::dump() means echo. If it's true then the dump result will be echoed, if it's false it will be returned as a string. In your case you need it to be false.

Suppose your array is

$a = array ('a' => 'apple', 'b' => 'banana', 'c' => array ('x', 'y', 'z'));

then you have to write below code to write proper array format in your log file

$this->_logger->log(100,print_r($a,true));

It will print in you log file

[2015-11-09 06:58:27] main.DEBUG: Array
(
    [a] => apple
    [b] => banana
    [c] => Array
        (
            [0] => x
            [1] => y
            [2] => z
        )

)
 {"is_exception":false} []