Ok, here is what I think is happening (according to the examples above):

$y = 42;
$x = $y;

substr( $string, 2, 6 ) = '';
$result = substr( $string, 2, 6 );
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The other ways of writing it, $x = $y = 42; and your example are just shorthand ways.

And perldoc specifically mentions the way when you use a fourth argument instead as an alternative where you can get the replaced values back, unlike the assignment way.

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