my $bar = ${exists $foo{'bar'} or \undef };

##</code><code>##

my $bar = $foo{'bar'};

##</code><code>##

my $bar = $foo{'bar'}{'baz'};

##</code><code>##

my $bar = ${exist $foo{'bar'}{'baz'} or \undef};

##</code><code>##

my $qux = \%foo;
$qux = exists $qux->{$_} or (undef $qux, last)
    for qw(bar baz quux qux);