Corion, CountZero and Anonymous Monk are all correct about turining on binmode for input stream and STDOUT. The following is how I would do something similar, it uses the File::Basename to extract the basename from a given path, open and read the file using IO::File, and output compliant header using CGI.

use strict;
use warnings;
use File::Basename;
use IO::File;
use CGI;

binmode STDOUT;

my $target_file = "/path/to/target_file.dat";

my $f = new IO::File "$target_file", "r" or die;
my $file = do { local $/; binmode $f; <$f> };

my $cgi = new CGI;
print $cgi->header( -type => 'application/x-unknown',
                    -attachment => basename($target_file) ),
      $file;
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