It is called "autovivification". Have a read through perlreftut and perlref.
You have seen it happen in a light weight way whenever you assign a value to an array element that hadn't been used before. It is a little more interesting in the case of a reference which you are using for the first time, but is one of the underpinnings of creating and managing "interesting" data structures in Perl, as indeed was the case here.
My pleasure, education is a large part of what PerlMonks is about.
DWIM is Perl's answer to Gödel
Are you posting in the right place? Check out Where do I post X? to know for sure.
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link
or How to display code and escape characters
are good places to start.