use strict;
use warnings;
use HTML::Template;

my $str = <<STR;
For example, if user defines in a template a string "%section_10%" I retrieve
value "10", make a mySQL query to select some text from row with id = 10 and
then replace text with query result "<!-- TMPL_VAR "section_10" -->".
STR

my @db = ('', qw(the quick brow fox jumps over the lazy dog should suffice));
my $template = HTML::Template->new (scalarref => \$str);
my %usedParams = map {lc ($_) => 1} $template->param ();
my @usedIds = map {m/section_(\d+)/; $1} keys %usedParams;

$template->param ("section_$_" => $db[$_]) for
    grep {defined $_ && length $db[$_]} @usedIds;

print $template->output ();

##</code><code>##

For example, if user defines in a template a string "%section_10%" I retrieve
value "10", make a mySQL query to select some text from row with id = 10 and
then replace text with query result "should".