Hello, working with this right now myself.

The first three parameters to the upload_hook() routine are supplied by CGI.pm. The last fourth parameter is optionally provided by you in the new() call, and is passed to the hook routine as a convenience for you.

• $filename - is the value from the file input form field, supplied by the browser, and thus might be just the remote filename or might be the whole remote path to the file
• $buffer - really is the current buffer contents; I thought it'd be at least a ref to the buffer, but it really is the whole 4KB thing (ugh); the data here will be written to CGI.pm's temp file; I'm not sure what use it is to look at it
• $bytes_read - this is the cumulative total number of bytes read so far; not just the number of bytes read into this buffer
• $data - this can be any value supplied by you in the call to new(), so you could use it anyway you wish; I use it to pass an object reference so that my upload_hook() routine can reference the master data structures

In my recent code I am doing

my $q = CGI->new( \ &WhizBang::upload_hook, $self );
# where $self is a ref to the current WhizBang object

# then later 
sub upload_hook {
  my  $remote_filename  = shift;
  my  $buffer           = shift;
  my  $bytes_read       = shift;
  my  $self             = shift;
  my  $progress = $self->param('progress');
  my( $filename ) = $remote_filename =~ m/^ (?: .* [:>\]\\\/] )? ( .* 
+) $ /xs;
  $progress->{file_name}  = $filename;
  $progress->{bytes_xfrd} = $bytes_read;
  $progress->{time_now}   = time();
  # other processing to output data to external storage
  # for display from another script
}
[download]

Elsewhere I grab the value in $ENV{CONTENT_LENGTH} to compare against the $bytes_read value.

I have a couple of quibbles about this recent addition to CGI.pm, but at least the optional last $data parameter makes finding and updating your 'home' data structure possible.

As far as what you can do with this, yes, you pretty much must capture the data and write it to some place that _another_ script can find it for display. Once CGI.pm starts processing the POST'ed data (when you call new()) it will not return until all nnnMB and nn files are done being received. I'm writing the progress information to a database for a Javascript spawned window to find.

Some of these recent posts have references to examples:
Upload Progress Bar
Deferred/selective processing in CGI.pm
Upload Progress
Re: Re: CGI.pm and Large File Transfers

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