$config->{$field} //= {};
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// is the Logical Defined-Or operator (see perlop), added with Perl version 5.10. So the example from the Camel book
    $val ||= "2"; # Set $val to 2 if it isn't already "true"
exactly translates to
    $val //= "2"; # Set $val to "2" if $val isn't defined