then the left side must be either a hard or symbolic 
reference to an array, a hash, or a subroutine respectively.
(Or technically speaking, alocation capable of holding a 
hard reference, if it's an array or hash reference being 
used for assignment.)
See the perlreftut manpage and the perlref manpage.

Otherwise, the right side is a method name or a simple 
scalar variable containing either the method name or a 
subroutine reference, and the left side must be either an 
object (a blessed reference) or a class name (that is, a 
package name). See the perlobj manpage.