In lvalue context,

$var->{$key}
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( $var //= {} )->{$key}
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defined( $cg->{mainsnak}{datavalue}{value}{notexist} )
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is equivalent to

defined( ( ( ( ( $cg //= {} )->{mainsnak} //= {} )->{datavalue} //= {}
+ )->{value} //= {} )->{notexist} )
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That could potentially create a lot of new hashes.

Solution 1:

if (           $cg
   &&          $cg->{mainsnak}
   &&          $cg->{mainsnak}{datavalue}
   &&          $cg->{mainsnak}{datavalue}{value}
   && defined( $cg->{mainsnak}{datavalue}{value}{notexist} )
) {
   ...
}
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Solution 2:

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