With mod_perl and Apache::File, this is rather easy:

use Apache::Constants qw(:common);
use Apache::Request;
use Apache::File;

sub handler {
  my $r = Apache::Request->new(shift);
  my $serverFile = $r->param('file');
  # needs some errorhandling here if $serverFile is not 
  # submitted, invalid paths, ...

  # get $contenttype somehow; I always save it in a
  # database when I upload the file
  
  my $fh = Apache::File->new($serverFile);
  binmode($fh);
  unless ($fh) {
    $r->log_error("Can't open '$serverFile': $!");
  } # unless
  else {
    $r->send_http_header($contenttype);
    $r->send_fd($fh);
  } # else
  return OK;
} # handler
[download]

well, you'll need to do some errorhandling...

If you use a temporary filename at the server, it is not so easy to tell the client to save the file under the original filename. I like to use mod_rewrite (httpd.conf) or PerlTransHandler to prevent this and which could look like:

RewriteEngine on
RewriteRule /download/(.+)/(.*) /perl-bin/download.pl?file=$1;$2
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There $1 is the temporary filename and $2 the original filename. Or with PerlTransHandler:

package TransHandler;
use warnings;
use strict;
use Apache::Constants qw(DECLINED);

sub handler {
  my $r = shift;
  my $standardUri = "/pboard/PBoard.pm";
  my $uri = $r->uri();
  if ($uri =~ m|^/download/([^/]+)/(.+)|) {
    $r->uri($standardUri);
    $r->args("action=download2;file=$1");
    return DECLINED;
  } # elsif

  return DECLINED;
} # handler
[download]