Works for me. Note you are not doing a HEAD request, but a GET. I changed your sub a little (moved the use statements to the top of the script).

sub simple_get {
    my $url = shift;
    my $ua = LWP::UserAgent->new;
    my $request = HTTP::Request->new(GET => $url);
    my $response = $ua->request($request);
    warn "ERROR!" unless $response->is_success;
    print $response->as_string;
}
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And this is the output–

200 OK
Server: Welcome to UniProt FTP service.
Content-Length: 151
Content-Type: text/plain
Last-Modified: Wed, 19 Mar 2014 14:00:00 GMT
Client-Date: Thu, 20 Mar 2014 17:25:38 GMT
Client-Request-Num: 1

UniProt Knowledgebase Release 2014_03 consists of:
UniProtKB/Swiss-Prot Release 2014_03 of 19-Mar-2014
UniProtKB/TrEMBL Release 2014_03 of 19-Mar-2014
[download]