Thanks everyone for your help,

using blokhead's first reply, I typed the following data structure, and passed it 'raw' to HTML::Template. 
%loop_data = ( rows => [ { cells => [ { data => 'one'},
                                      { data => 'ein'},
                                     ],
                          },
                          { cells => [ { data => 'two'},
                                       { data => 'zwei'},
                                      ],
                          },
                          { cells => [ { data => 'three'},
                                       { data => 'drei'},
                                      ],
                          },
                        ],
                );

[download]
Being able to see this raw data structure helped me to figure out how to create it programatically for HTML::Template.

I was now able to see how cfeaks's "foreach" example worked.

then... thanks to jeffa's replies and his Map: The Basics tutorial, I was able to figure out what the map statements were doing.

Using this new-found knowledge, I was able to write this new script which uses an array '@columns' to 1/, fill one loop (the first table row) with values, and 2/ create the rest of the table by filling a loop-within-a-loop with data from a database.
#!/usr/bin/perl -wT
use strict;
use CGI;
use DBI;
use HTML::Template;

use vars qw/ $q $dbh $sth $rv $rc
             $template $html @columns @headings_data @rows_data $sql /
+;

@columns = qw/ prod_code dep_1 dep_2 dep_3 /;

@headings_data = map {{ cell_data => $_ }} @columns;

$sql = 'SELECT ' . join(', ', @columns) . ' FROM products WHERE dep_1=
+01';

$dbh = DBI->connect("dbi:mysql:database:host", "user", "pass")
  or print_error("Could not connect to Database", $DBI::errstr);

$sth = $dbh->prepare( $sql )
  or die("Couldn't prepare the database", "$DBI::errstr");

$rv = $sth->execute(@_)
  or die("Couldn't execute database", "$DBI::errstr");

while ( my @data = $sth->fetchrow_array ) {
  my @cells = map {{ cell_data => $_ }} @data;
  push @rows_data, { cells => \@cells };
}

$html     = do { local $/; <DATA> };
$template = HTML::Template->new(scalarref => \$html);
$template->param(  headings => \@headings_data,
                   rows     => \@rows_data
                );

$q = new CGI;
print $q->header;
print $template->output;

$dbh->disconnect;

exit;


__DATA__
<html>
<body>
<table>
<tr>
  <!-- TMPL_LOOP NAME=headings -->
  <td><!-- TMPL_VAR NAME=cell_data --></td>
  <!-- /TMPL_LOOP -->
</tr>
<!-- TMPL_LOOP NAME=rows -->
<tr>
  <!-- TMPL_LOOP NAME=cells -->
  <td><!-- TMPL_VAR NAME=cell_data --></td>
  <!-- /TMPL_LOOP -->
</tr>
<!-- /TMPL_LOOP -->
</table>
</body>
</html>

[download]
The original data I posted was not in the format I was actually working with, but I presented it as such to enable me to figure out exactly what HTML::Template wanted, and how to create that myself (rather that using another-monks map-statement-on-a-plate that I couldn't understand).
A big thankyou to everyone for having the patience to help me past another milestone of Perl understanding!

In reply to Re: (fireartist)HTML table with HTML::Template and loop in a loop by fireartist
in thread HTML table with HTML::Template and loop in a loop by fireartist

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.