my %hash = (
    a => {
        ab => {
            ac => 'yup',
        },
    },
);

# Check for existence
if (exists $hash{aa}->{ab}->{ac}) {
    ## Notice that 'aa' is a typo an doesn't exist
    ## but will be auto-created and now contain a 
    ## hash ref with one entry 'ab'.
    print "ac exists.\n";  ## won't get printed
}
if (exists $hash{aa}) {
    ## This will succeed because $hash{aa} was auto
    ## created above.
    print "aa exists.\n";  ## will get printed
}

##</code><code>##

if exists
    $hash{aa}  ## exists doesn't check this
    ->{ab}     ## Nor this
    ->{ac}     ## Ah ha!  exists does check here
;