W::M is basically LWP::UserAgent. so you can also refer to perldoc lwpcook the 'large documents' section.

LARGE DOCUMENTS
    If the document you want to fetch is too large to be kept in memory,
    then you have two alternatives. You can instruct the library to write
    the document content to a file (second $ua->request() argument is a file
    name):

      use LWP::UserAgent;
      $ua = LWP::UserAgent->new;

      my $req = HTTP::Request->new(GET =>
                    'http://www.linpro.no/lwp/libwww-perl-5.46.tar.gz');
      $res = $ua->request($req, "libwww-perl.tar.gz");
      if ($res->is_success) {
         print "ok\n";
      }
      else {
         print $res->status_line, "\n";
      }

    Or you can process the document as it arrives (second $ua->request()
    argument is a code reference):

      use LWP::UserAgent;
      $ua = LWP::UserAgent->new;
      $URL = 'ftp://ftp.unit.no/pub/rfc/rfc-index.txt';

      my $expected_length;
      my $bytes_received = 0;
      my $res =
         $ua->request(HTTP::Request->new(GET => $URL),
                   sub {
                       my($chunk, $res) = @_;
                       $bytes_received += length($chunk);
                       unless (defined $expected_length) {
                          $expected_length = $res->content_length || 0;
                       }
                       if ($expected_length) {
                            printf STDERR "%d%% - ",
                                      100 * $bytes_received / $expected_length;
                       }
                       print STDERR "$bytes_received bytes received\n";

                       # XXX Should really do something with the chunk itself
                       # print $chunk;
                   });
       print $res->status_line, "\n";