As BUU and the others have already stated ... this is working exactly as it should. If your test.pl file actually returns a plain text header and VALID JavaScript. You don't need to change your HTML, just the Perl script:

#!/usr/bin/perl -T

use strict;
use warnings;
use CGI qw(:standard);

print header('text/plain'),
   'window.document.title = "Acesta este continutul fisierului"'
;
[download]

But what this buys you beats me ... seems very silly.