$ perl -Mstrict -Mwarnings -MData::Dump -le '
    my %x = (a => { B => 2 });
    
    for my $key (qw{a b}) {
        print join "^" => map { $x{$key}{$_} || "undef" } qw{B C};
        dd \%x;
    }
'
2^undef
{ a => { B => 2 } }

undef^undef
{ a => { B => 2 }, b => {} }