You need 2 things: a form and a script that processes it. Actually you can also have the script create the form, which lets you have anything in one place.

Below is a simple example. It uses CGI to process generate and process the form, and XML::Simple to turn the parameters into XML. You obviously need to have the script setup so it can run as a cgi on your server. Also the data should not be in the web tree, and beware of the security implications of letting an outside client create data on your machine.

#!/usr/bin/perl -w
use strict; 

use CGI qw/:standard :cgi-lib/;
use XML::Simple;

# the web server needs create and write access to that file
# it should probably not be in the web tree (under DocumentRoot
# or any accessible directory like cgi-bin and al. under Apache)
my $OUT= "/web/data/out.xml";

# dispatch table <function param> => <sub that processes it>
my %process= ( display_form => \&display_form,
               process_form => \&process_form,
             );

my $function= param( 'function') || 'display_form'; # display_form is 
+the default
Delete( 'function'); # we don't need to save function in the XML, so l
+et's remove it

if( my $process= $process{$function})
  { $process->(); }
else
  { display( p( h2( "Error")), p( "wrong function '$function'") ); }

exit;

# the interesting part, gets the params and saves them as XML
sub process_form
  { # from CGI          get all the params   
        my %params = Vars;

        # only useful if you have checkbox_group (that can take severa
+l values
        # for a single param). There might be a better way, but this s
+houd work
        foreach my $param (%params)
          { next unless( defined $params{$param});    # to avoid warni
+ngs when the param is not filled
                my @values= split("\0", $params{$param}); # multiple v
+alues are separated by a null char
                if( @values > 1) { $params{$param}= [ @values ] }
            }
        
    # from XML::Simple  turn them into xml read the docs!
    my $xml=  XMLout( \%params, noattr => 1, rootname => 'data', xmlde
+cl => 1 ); 

        # I am being extra-paranoid here, but you never know
        # notethat this sends an Internal Error to the client 
    open( OUT, '>', $OUT) or die "cannot open $OUT: $!";
    print( OUT $xml)      or die "could not print in $OUT: $!";
    close OUT             or die "could not close $OUT: $!";
        
    # send back an acknowledgement and the data (escaped, in pre tags)
    display( p( "OK, XML data saved in $OUT"), pre( escapeHTML( $xml))
+);
  }

# this is just an example, the generated form can be in a separate fil
+e,
# or generated using a templating module (see Text::Template or HTML::
+Template)
# the form here is straight out of the CGI docs
sub display_form
  { display( h1('A Simple Example'),
               start_form,
                             hidden( function => 'process_form'),
               "What's your name? ",textfield('name'),p,
               "What's the combination?", p,
               checkbox_group(-name=>'words',
                              -values=>['eenie','meenie','minie','moe'
+],
                              -defaults=>['eenie','minie']), p,
               "What's your favorite color? ",
               popup_menu(-name=>'color',
                          -values=>['red','green','blue','chartreuse']
+),p,
               submit,
               end_form,
               hr
         )
  }


# a helper sub that wraps the text to be displayed 
sub display
  { print header,                          # HTTP header
          start_html('A Simple Example'),  # title
          @_,                              # what we want to display
          end_html;
  }
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