Whenever you get errors or warnings, I suggest you look up the perldiag manual page to see more verbose explanation. Or, you can put the use diagnostics; line near the top of your program. This way, you let Perl explains more detail about what's wrong or what might be wrong in your program. The first is really a warning message instead of an error (indicated by "(W)", while the second is a fatal error.

Use of reference "%s" as array index
  (W misc) You tried to use a reference as an array index; this probab
+ly
  isn't what you mean, because references in numerical context tend to
+ be
  huge numbers, and so usually indicates programmer error.

  If you really do mean it, explicitly numify your reference, like so:
  $array[0+$ref].  This warning is not given for overloaded objects,
  either, because you can overload the numifi- cation and stringificat
+ion
  operators and then you assumedly know what you are doing.

Can't call method "%s" on unblessed reference
  (F) A method call must know in what package it's supposed to run.  I
+t
  ordinarily finds this out from the object reference you supply, but 
+you
  didn't supply an object reference in this case.  A reference isn't a
+n
  object reference until it has been blessed.  See per- lobj.
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The @unsorted array contains list of objects of GARD::Book class, when you pass it to the sort function, the $a and $b variables contain references to the correspondent elements in that array. So you simply say $a->function instead of $unsorted[$a]->function in the sort block.

Saying $a->function in the sort block is the same with $unsorted[SOME_INDEX]->function outside the block. When you say $unsorted[$a]->function in the sort block the following (more or less) happens:

• $a becomes a reference of one of @unsorted element, which is an object of GARD::Book
• Now you're using that reference (stringified as GARD::Book=HASH(0x2b7831c)) as array index, but, usually, this is not something you want, hence the warning. Imagine that you're actually doing $unsorted["GARD::Book=HASH(0x2b7831c)"]->function instead of $unsorted[0]->function.
• The $unsorted[$a] is evaluted to some reference and used to call a method. But we need more than a mere reference to call a method, we need a blessed reference, and that's what object is all about in Perl (see the first few lines in the perlobj). That's what triggered the fatal error: unblessed reference was used to call a method.