The enctype="multipart/form-data" bit is important, without this the file won't get uploaded. I've never used WWW::Mechanize but with CGI you should be able to recover the file size, type, filename and contents using something like this (untested) code:

use CGI;
my $query = new CGI;
my @names = $query->param;
foreach my $name (@names) {
  print "$name => ".$query->param($name)."\n";
}
[download]

Also note that the CGI module has methods for creating the upload form, and make sure you read about how to limit file sizes.