if (can_send($file)) { # we assume you can program whatever rules you 
+need here
  open(INP,"<$file") || die "Can not open $file:$!\n"; # but  handle t
+hat in can_send
  my MIME::Types $types = MIME::Types->new;
  my $mime = $types->mimeTypeOf($file) || 
      "application/octet-stream";
  print header("Content-type: $mime");
  binmode(INP);
  binmode(STDOUT);
  while (<INP>) {
    print $_;
  }
  close(INP);
}
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That works, but I don't like the use of the <> diamond operator to deal with binary files. If there aren't any newline characters in the file, you'll read it all in one go; if there are many of them you'll read a line and print it many times.

I think it's better to use the simple read function for this:

while (read(INP,$buf,8192)) {
  print $buf;
}
[download]

Much more predictable behavior.

Well, if you use sysread, you should also use syswrite instead of print. That way you don't have to bother with binmode, etc. And somehow, it seems cleaner to me than mixing the two different idioms.

CGI::header(
    -type => "application/octet-stream; name=$filename",
    '-Content-Disposition' => "attachment; filename=$filename",
    '-Content-Transfer-Encoding' => 'binary',
);
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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
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