Using the aforementioned HTML::Template is a good solution to repopulating an HTML form after Perl-side validation. I do this all day long. H::T has a built-in function that preserves the values from the form:

...
my $query = new CGI;
...
my $template = HTML::Template->new( filename => "foobar.tmpl",
                                            associate => $query);
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However, one problem in any of the methods described above, is resetting radio buttons, check boxes, or select/option dropdowns. Besides using the associate function in H::T, you need to set your returning params to trigger which buttons, boxes, or dropdowns have been selected. Here's my messy solution (I'm open to new ways, monks):

Perl:
my $vote = $query->param('vote');
my ($voteyes, $voteno);

if ($vote == 1) { 
   $voteyes == 1; 
} elsif ($vote == 2) {
   $voteno == 1;
}

$template = param(voteyes => $voteyes,
                                 voteno => $voteno); 

HTML:
Yes: <input type="radio" name="vote" value="1"<tmpl_if voteyes> checke
+d</tmpl_if> />
No: <input type="radio" name="vote" value="2"<tmpl_if voteno> checked<
+/tmpl_if> />
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Just a friendly "heads-up." Good luck.