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$var->{$key}

( $var //= {} )->{$key}

defined( $cg->{mainsnak}{datavalue}{value}{notexist} )

defined( ( ( ( ( $cg //= {} )->{mainsnak} //= {} )->{datavalue} //= {}
+ )->{value} //= {} )->{notexist} )

if (           $cg
   &&          $cg->{mainsnak}
...
) {
   ...
}

no autovivification;
if (defined( $cg->{mainsnak}{datavalue}{value}{notexist} )) {
   ...
}