In essence, $node_array_ref is a reference to an array; it is a single scalar value. @$node_array_ref (which you might see written as @{ $node_array_ref }) dereferences $node_array_ref to produce an array of scalar values. See perlreftut for a short tutorial about references.

The main Perl link that I have bookmarked is http://perldoc.perl.org/perl.html. This has links to all the Perl documentation. I'd recommend you start with perlintro, then look at the links under Tutorials and then move on to the Reference Manual. There is a vast amount of information here: don't try to learn everything at once; instead, get a feel for which sections contain which information and revisit often - you'll build up a solid body of core knowledge with the ability to quickly find specifics when you need to.