The two are identical: perl specifically allows $x{a}{b} as shorthand for $x{a}->{b}. Which you use is a matter of personal preference or house style.

Note that this shorthand is available only for the second (or subsequent) dereference: $x{a} and $x->{a} are different, the first looking up a key in the hash %x while the second looks up a key in the hash reference $x.