I think that the problem was that the HDSS system isn't currently available:-). This worked:

#!/usr/bin/perl -w

use strict;
use HTTP::Lite;

my $req;
my $ncdc = new HTTP::Lite;
my $format = 'NEXRAD Level II';
my $dataset = '6500';
my $radar_site = 'KOAX';
my $year = '2001';
my $email = 'deadpickle@gmail.com';

my %vars = ("stations" => $radar_site,
            "station_lst" => "",
            "typeofdata" => "RADAR",
            "dtypelist" => "",
            "begdatestring" => "",
            "enddatestring" => "",
            "begyear" => $year,
            "begmonth" => '01',
            "begday" => '01',
            "beghour" => "",
            "begmin" => "",
            "endyear" => $year,
            "endmonth" => '12',
            "endday" => '31',
            "endhour" => "",
            "endmin" => "",
            "timeselectminlist" => "",
            "timeselectbuffer" => "",
            "gvartype" => "",
            "outmed" => "FTP",
            "outpath" => "",
            "pri" => "500",
            "altdsname" => "",
            "altstations1" => "",
            "altstations2" => "",
            "datasetname" => $dataset,
            "emailadd" => $email,
            "directsub" => "N",
            "outdest" => "FILE",
            "applname" => "",
            "subqueryby" => "STATION",
            "tmeth" => "Awaiting-Data-Transfer",
            );
            
    $ncdc->prepare_post(\%vars);
    $req = $ncdc->request("http://www.ngdc.noaa.gov/metadata/published
+/NCDC/C00345.xml") or die "Unable to get document: $!";
    print "req: $req\n";
    print $ncdc->body();
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