Classic solution is to use Data::Diver

use Data::Diver qw/ Dive /;
##       if (defined $cg->{'mainsnak'}->{'datavalue'}->{'value'}->{'no
+texist'}) {}  ## THIS LINE
       if ( defined Dive( $cg, qw' mainsnak datavalue value notexist '
+ ) ) {
            
       }
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Ex

$ perl -e "use Data::Diver qw/Dive/; $f{a}{b}{c}=666; print Dive(\%f, 
+qw/a b c Q/); print Dive(\%f, qw/a b c/); "
666
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