>perl -wMstrict -le
"my %hash = qw(a 1 b 2);
 print 'true 1st level' if $hash{c};
 print exists $hash{c} ? '' : 'NOT ', 'exists 1 c';
 print 'true 2nd level' if $hash{c}{d};
 print exists $hash{c}    ? '' : 'NOT ', 'exists 2 c';
 print exists $hash{c}{d} ? '' : 'NOT ', 'exists 2 d';
"
NOT exists 1 c
exists 2 c
NOT exists 2 d