Technically, this is an Apache question, and not a Perl one. But if you must do it with Perl, here's something to get you started:

#!/usr/bin/perl -wT

use CGI;
use File::MimeInfo qw( mimetype );
use CGI::Carp qw( fatalsToBrowser ); #Of course, you'll want a more el
+egant method
                                     # of error reporting than I use h
+ere
use constant FILES_DIR => "/path/to/files/";

my $cgi = new CGI;


#Get the requested filename
my $wanted_file = $cgi->param( "file" );
#my $wanted_file = $cgi->keywords;

#Check if data is tainted - VERY IMPORTANT
unless( $wanted_file =~ /\w{1}[\w-.]*\@[\w-.]+/ ) {
    #User tried to pass tainted data, e.g. "/etc/passwd"
    die( "Nice try, buddy." );
}

#Check if that actually exists
unless( -f FILES_DIR.$wanted_file" ) {
    die( "That file cannot be found." );
}

#Check if the user is allowed to see that file
#NOTE: Using cookies to store user id's is a bad idea. You
#       should use a session or a token of some sort.
my $given_userid = $cgi->cookie( "user_id" );
my @allowed_people = get_an_array_of_allowed_people_from_somewhere();
unless( grep($given_userid, @allowed_userid) ) {
    die( "You are not allowed to see this file." );
}

#Since we got here, we can give the user their file
my $content_type = mimetype( FILES_DIR.$wanted_file );
open( FILE, "<", FILES_DIR.$wanted_file ) or die( "Cannot open file: $
+!" );
print $cgi->header( -type => $content_type );
print while <FILE>;
close FILE;
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