It's just like $hash{foo}, except you have multiple keys.

@hash{qw( foo bar )} = (1, 2);
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is basically the same as

$hash{foo} = 1;
$hash{bar} = 2;
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(I said "basically" because there is a difference. The former uses a list assignment while the latter uses two scalar assignments. List assignments don't return the same as scalar assignments. See Mini-Tutorial: Scalar vs List Assignment Operator)