$ perl -MData::Dumper -e 'my $h; $h->{x}++ if exists $h->{x}; print Dumper($h)'
$VAR1 = {};
$ perl -MData::Dumper -e 'my $h; $h->{x}++ if defined $h->{x}; print Dumper($h)'
$VAR1 = {};

##</code><code>##

if ($h and exists $h->{x})

##</code><code>##

if ($h and exists $h->{a} and exists $h->{a}{b} and exists $h->{a}{b}{foo})