First, the most important difference between C pointers and Perl references is that there is no pointer arithmetic in Perl.

Second, in answer to your questions:

  1. The @_ array is special in Perl. On entry to a sub, it contains aliases to the arguments that were passed; $_[2] = 42; will set whatever variable was used as the third argument to 42. On line 28 of your code, $pointer_to_diff will itself be set to the difference; note that printing it emits an integer instead of a reference debugging value. On line 31, you pass a newly-constructed reference to $diff instead; that reference is itself an SV, so its value gets overwritten just before it is discarded. The $diff variable itself is never actually set. Devel::Peek is likely to be helpful here.
  2. As an lvalue, it means to assign to the variable to which the reference points. Since $res is itself a reference to the alias of the parameter that was passed in, this will replace the value that was passed in. On line 28, that is the $pointer_to_diff variable. On line 31, that is an anonymous temporary constructed to hold a reference to $diff.
  3. The earlier two answers answer this as well.

Lastly, read the documentation, particularly, perlreftut, perlref, and perldsc.

In reply to Re: Pointers and References by jcb
in thread Pointers and References by Leudwinus

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.