$ cat lwp-headers.pl
#!/usr/bin/perl

package MyLWP;
use base 'LWP::UserAgent';

sub progress {
    my($self, $status, $resp) = @_;
    if ($resp) {
        my @headers = $resp->header_field_names;
        print "response headers: @headers\n";
    }
}

package main;

my $ua = MyLWP->new;
my $url = shift || 'http://gmail.google.com';
my $rp = $ua->get($url);
print $rp->status_line, "\n";

$ ./lwp-headers.pl http://www.google.com
response headers: Cache-Control Connection Date Location Content-Length Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Location Content-Length Content-Type Client-Date Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Content-Type Client-Date Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
200 OK

$ ./lwp-headers.pl
response headers: Cache-Control Connection Date Pragma Location Content-Type Client-Peer Client-Response-Num Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Pragma Location Content-Type Client-Date Client-Peer Client-Response-Num Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Pragma Location Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Pragma Location Content-Type Client-Date Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Pragma Location Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Pragma Location Content-Type Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Pragma Location Content-Type Client-Date Client-Peer Client-Response-Num Set-Cookie Title X-Cache X-Cache-Lookup
response headers: Cache-Control Connection Date Pragma Server Content-Length Content-Type Client-Peer Client-Response-Num Client-SSL-Cert-Issuer Client-SSL-Cert-Subject Client-SSL-Cipher Client-SSL-Warning Set-Cookie
.....
response headers: Cache-Control Connection Date Pragma Server Content-Length Content-Type Client-Date Client-Peer Client-Response-Num Client-SSL-Cert-Issuer Client-SSL-Cert-Subject Client-SSL-Cipher Client-SSL-Warning Set-Cookie Title
200 OK