Have you read the documentation of the CGI module which I linked to before? It handles the reading of POST parameters for you and gives you a nice, convenient file handle to read the uploaded file data from. I recommend searching the documentation for creating a file upload field. Note that you don't need to use the output methods of CGI and can still produce the HTML using print. But reading the uploaded data would likely be done by:

use CGI '';
my $q = CGI->new();
my $uploaded_file = $q->param('t'); # "t" is the name of the upload fi
+eld;
...
[download]

But looking at your posted HTML code, it makes no sense - you use the name t for both, the uploaded file and also for the submit button. While that's possible, it makes things harder than neccessary. Change one of the two to a different name.