use strict;
use warnings;
use base 'CGI::Application';
use CGI::FormBuilder;

main->new->run; # emulated for example purpose

sub setup {
    my $self = shift;
    $self->start_mode('create_form');
    $self->run_modes([qw(create_form list)]);
}

sub create_form {
    my $self = shift;
    my @fields = qw(name age);
    my $cgi = $self->query;

    my $form = CGI::FormBuilder->new(
        fields => \@fields,
        params => $cgi,
        keepextras => 1,
    );

    # or, instead of using the keepextras option
    # you call the method with the same name
    # $form->keepextras(1);

    # push onto the CGI parameter stack
    $cgi->param($self->mode_param => $self->get_current_runmode);
    # UPDATE: change hardcoded "rm" to method call

    print $form->submitted && $form->validated
        ? $form->confirm
        : $form->render;
}

##</code><code>##

$ perl cfb.pl
Content-Type: text/html; charset=ISO-8859-1


<!-- Generated by CGI::FormBuilder v3.03 available from www.formbuilder.org -->
<form action="cfb.pl" method="get">
<div>
<input id="_submitted" name="_submitted" type="hidden" value="1" /><input id="rm" name="rm" type="hidden" value="create_form" />
<table>
<tr valign="top">
  <td>Name</td>
  <td><input id="name" name="name" type="text" /></td>
</tr>
<tr valign="top">
  <td>Age</td>
  <td><input id="age" name="age" type="text" /></td>
</tr>
<tr valign="top">
  <td align="center" colspan="2"><input id="_submit" name="_submit" type="submit" value="Submit" /></td>
</tr>
</table>
</div>
</form>