use Data::Diver qw/ Dive /;
##       if (defined $cg->{'mainsnak'}->{'datavalue'}->{'value'}->{'notexist'}) {}  ## THIS LINE
       if ( defined Dive( $cg, qw' mainsnak datavalue value notexist ' ) ) {
            
       }


##</code><code>##

$ 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