I needed a two-dimensional matrix, and what emerged from my sleep-deprived fingertips was: 

    my $matrix = [([(0) x $cols]) x $rows];

[download]

See the problem? I didn't, at first, until my code started behaving in wacky ways.

Data::Dumper revealed: 
$VAR1 = [
          [
            0,
            0
          ],
          $VAR1->[0]
        ];

[download]
The second row is a copy of the first, which is exactly what the x is supposed to do, but isn't what I'd expected. D'oh.

Since my first few test cases used a 1x1 matrix (start small, take small steps), I didn't notice the problem until later, when tests that stored data into a 2x2 matrix started acting wierd.

Having Data::Dumper around when doing test-driven development can be very handy.

In reply to Using x to build data structures considered harmful by dws

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.