If you have firefox with the livehttpheaders extension you can see the headers of all requests.

you can also use the lwp-request command that comes with LWP. For example:

lwp-request -m HEAD http://localhost/
[download]

gives me:

200 OK
Connection: close
Date: Wed, 19 Jul 2006 21:39:32 GMT
Accept-Ranges: bytes
ETag: "1cc17e-100e-3dba5c6e"
Server: Apache/1.3.33 (Debian GNU/Linux) PHP/4.3.10-16 mod_auth_pam/1.
+1.1 mod_perl/1.29
Content-Length: 4110
Content-Type: text/html; charset=iso-8859-1
Last-Modified: Sat, 26 Oct 2002 09:12:14 GMT
Client-Date: Wed, 19 Jul 2006 21:39:32 GMT
Client-Peer: 127.0.0.1:80
Client-Response-Num: 1
[download]

The important part in this case is the Content-Type header. For a stylesheet it should say Content-Type: text/css optionally followed by a semi-colon and charset value - and a lwp-request <url> should give you the content of the stylesheet.

You could also try to see if making a plain HTML page containing the output of your script gives the same problem. Trying different browsers can also be enlightening.