use CGI;
use CGI::Ex::Fill qw(fill);

my $q = CGI->new;
my %default = (
    foo => 'FooFoo',
    bar => 'BarBar',
    baz => 'BazBaz',
);

foreach my $k (keys %default) {
    $q->param($k, $default{$k})
        if ! defined $q->param($k);
}

my $html = '
<form>
<input type="text" name="foo">
<textarea name="bar"></textarea>
<select name="baz">
<option>Bing</option>
<option>BazBaz</option>
<option>Bang</option>
</form>
';

fill({text => \$html, form => $q});

print $html;


##</code><code>##

<form>
<input type="text" name="foo" value="FooFoo">
<textarea name="bar">BarBar</textarea>
<select name="baz">
<option>Bing</option>
<option selected="selected">BazBaz</option>
<option>Bang</option>
</form>