print "Content-Type: application/octet-stream\n\n";
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You can only use a single CRLF character as header separator. Here, you use two. Two CRLF character is used to separate headers from content.

After all, how can you have two Content-Type header? If what you have is a plain text file to send, you don't need that octet-stream. I think you want,

print "Content-Type: text/plain\n";
print "Content-Length: $filesize\n";
print "Content-disposition: attachment; filename=$filename\n\n";

open FILE, $filename or die "can't open $filename: $!\n";
print while <FILE>;
close FILE;
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Using \n as CRLF can be problematic. It's safer to use CGI or CGI::Simple, CRLF is handled automatically.

use CGI::Simple;
use strict;
use warnings;

my $cgi = CGI::Simple->new;
my $filename = '/usr/local/..../file.txt'; # assumed exists

print $cgi->header(
  -type => 'text/plain',
  -attachment => $filename,
  -content_length => -s($filename),
);

open FILE, $filename or die "Can't open $filename: $!\n";
print while <FILE>;
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