As for the CGI.pm, there are hints into it on how to create an HTML form to upload a file, but what about how to update this very file?

Keep reading. It's all there in the docs.

When the form is processed, you can retrieve the entered filename by calling param():

       $filename = $query->param('uploaded_file');
[download]

Different browsers will return slightly different things for the name. Some browsers return the filename only. Others return the full path to the file, using the path conventions of the user's machine. Regardless, the name returned is always the name of the file on the user's machine, and is unrelated to the name of the temporary file that CGI.pm creates during upload spooling (see below).

The filename returned is also a file handle. You can read the contents of the file using standard Perl file reading calls:

    # Read a text file and print it out
    while (<$filename>) {
       print;
    }

    # Copy a binary file to somewhere safe
    open (OUTFILE,">>/usr/local/web/users/feedback");
    while ($bytesread=read($filename,$buffer,1024)) {
       print OUTFILE $buffer;
    }
[download]

However, there are problems with the dual nature of the upload fields. If you use strict, then Perl will complain when you try to use a string as a filehandle. You can get around this by placing the file reading code in a block containing the no strict pragma. More seriously, it is possible for the remote user to type garbage into the upload field, in which case what you get from param() is not a filehandle at all, but a string.

To be safe, use the upload() function (new in version 2.47). When called with the name of an upload field, upload() returns a filehandle, or undef if the parameter is not a valid filehandle.

     $fh = $query->upload('uploaded_file');
     while (<$fh>) {
       print;
     }
[download]

This is the recommended idiom.

When a file is uploaded the browser usually sends along some information along with it in the format of headers. The information usually includes the MIME content type. Future browsers may send other information as well (such as modification date and size). To retrieve this information, call uploadInfo(). It returns a reference to an associative array containing all the document headers.

       $filename = $query->param('uploaded_file');
       $type = $query->uploadInfo($filename)->{'Content-Type'};
       unless ($type eq 'text/html') {
      die "HTML FILES ONLY!";
       }
[download]

Perl Training in the UK <http://www.iterative-software.com>

Hi and thanks for your help
In the mean time, I have found some useful modules to handle uploads. I also have found out how to use the open(HANDLE, "> /cgi-bin/test.dat"); statement.

Obviously, the problem I am now facing is that my server runs under NT (alas) and I am having trouble with file paths. Namely:
If I use the following statement: open(HANDLE, "> test.dat"); my new file test.dat is created on the root of the server where my perl script is running. This is not very nice, so I'm trying to put this file in a more usable location. For instance by using open(HANDLE, "> /cgi-bin/my_programm_folder/test.dat");

But it won't work. I have tried with backslashes, slashes, dots, double slashes, drive names, full paths, relative paths, it doesn't work. I come from the UNIX (and previously Apple) world and I don't know Windows paths very well, but what I have tried is supposed to be right!

What am I doing wrong?

#!/usr/bin/perl

use CGI;
use IO::File;
use POSIX;
use strict;

my $cgi = CGI->new;

#   temporary file name is supplied by POSIX::tmpnam function
my $buffer;
my $tmpfile = POSIX::tmpnam;

#   loop through CGI handle and write new temporary file
my $fh = IO::File->new(">".$tmpfile);
print $fh $buffer while (read($cgi->param('inputfile'), $buffer, 1024)
+);
$fh->close;

#   retrieve file name from CGI file field parameter
my $newfile = ($cgi->param('inputfile') =~ /^((?:.*[:\\\/])?)(.*)/s)[1
+];

#   rename file unless file exists
rename $tmpfile, $newfile unless -e $newfile;

#   return HTML headers and page data

.
.

exit 0;
[download]

Ooohhh, Rob no beer function well without!