perl -MData::Dumper -e 'my %h; print "yes\n" if exists $h{d}{j}; print Dumper \%h;'
$VAR1 = {
          'd' => {}
        };

##</code><code>##

if(defined $h{d} && exists $h{d}{j}) {...}

##</code><code>##

my %hash;
$hash{d}{j} = 'foo';

##</code><code>##

my %hash;
$hash{d} ||= {};
$hash{d}{j} = 'foo';