Using the HEAD command that comes with LWP, you can make a HEAD HTTP request which might send back a Last-Modified header. For example (from the shell):

% HEAD http://ax9.org/index.htm
200 OK
Connection: close
Date: Thu, 07 Mar 2002 04:03:14 GMT
Accept-Ranges: bytes
ETag: "9046a85-233-3c64116d"
Server: Apache/1.3.22 (Unix) mod_perl/1.26 mod_ssl/2.8.5 OpenSSL/0.9.6
+b
Content-Length: 563
Content-Type: text/html
Last-Modified: Fri, 08 Feb 2002 17:57:01 GMT
Client-Date: Thu, 07 Mar 2002 04:03:14 GMT
Client-Response-Num: 1

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