$ perl -wE'
my %hash = ( a => 1 );

if (defined $hash{b}) { 1 }
say "defined b: ",keys %hash;
if (exists $hash{c}) { 1 }
say "exists c: ", keys %hash;

if (defined $hash{B}->{foo}) { 1 }
say "defined deref B: ", keys %hash;
if (exists $hash{C}->{foo}) { 1 }
say "exists deref C: ", keys %hash;
'
defined b: a
exists c: a
defined deref B: aB
exists deref C: aCB