I've badly explained the structure. A script, let's say a.pl, produce a dinamycally page that, among other things, needs some dinamically generated Javascript code. So a.pl has these lines:

-script => {
    -language => 'Javascript',
    -src => 'my_script.pl'
}
[download]

in order to call my_script.pl. So the text/plain header is written by my_script.pl: it produces Javascript code, indeed. Maybe I could tell you only about my_script.pl, that is what's causing the problem, but I thought it was better to specify all the structure of script. I hope I've explained things more clearly now. Thank you