One thing to keep in mind as you study this aspect of Perl is the notion of references, as discussed in the two perldoc pages you were given earlier.   Read these slowly and carefully, because “references” are used a lot when dealing with hashes.   (In fact, a so-called “hashref” is more-common than an actual variable that is a hash.   References are very versatile.   The contents of a hash are often references, this being the actual mechanism that gives us “hash of hashes” and other arbitrarily-complex data structures.)

A reference is a value that “refers to” another piece of data ... a piece of data which might be “anonymous” in that the only way to reach it is through a reference.   More than one reference can be made to the same piece of data, and your troubles will begin if you don’t realize that this is what Perl is actually doing.   This can lead to confusion like:   “Why did $b also change, when I only meant to change $a?”   (Answer: “because both variables are references to the same piece of data ... whether you intended for them to be or not.”)   If there is any one thing that confused me the most when I was getting started with this language, this was probably it.