It could be because the emusic webserver specifically doesn't allow the HEAD request, but does pass a Content-Length on a GET. That would explain why Netscape knows the length of the mp3s. If that is the case, the $response->headers will know the Content-Length after a GET.

I've seen several webservers do this, possibly to make it more difficult to crawl their sites. I have neither an emusic account nor LWP installed here, but I remember trying to make a crawler and running into this problem.

But, as mentioned in another writeup, if the file is being passed by a lame script, there might not be any Content-Length field at all.

If you want to know the size while downloading, you should probably use a callback when doing the request and pass the $response object to the callback. (And then have the callback die if the size isn't acceptable, I guess).

But even if you do get a Content-Length from either HEAD or GET, some webservers may return a wrong Content-Length or 0 as most browsers apparently only use this for displaying some information.