=()= is not an operator. 
$scalar =()= EXPR

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is a funky way of writing 
$scalar = ( () = EXPR )

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It consists of two operations: a list assignment (() = EXPR) as the RHS of a scalar assignment ($scalar = ...). Both types of assignments are documented in perlop, but the bits that apply are:

In short, it's a means of evaluating an expression in list context and returning the number of elements returned instead of the elements themselves.

See Mini-Tutorial: Scalar vs List Assignment Operator

In reply to Re: meaning of =()= by ikegami
in thread meaning of =()= by Anonymous Monk

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