I get this error message: Error reading Upload progress... which appears to be that hook is not starting I think.

I think it means the opposite: your hook is called but fails to read the session file. Your code looks like this...

if (-f "./$sessid.session") {
    # Read it.
    open (READ, "./$sessid.session");
    my $data = <READ>;
    close (READ);
    print "$data";
} else {
    print "Error reading Upload progress...";
}
[download]

... so your error message is telling you that "./$sessid.session" is not a plain file. So the file isn't there or you don't have permission to read it or $sessid has a garbage value.

Also, you should check the return value of open and I would ditch the use of -f and just try to open the file instead...

if ( open my $read_fh, '<', "./$sessid.session" ) {
    my $data = <$read_fh>;
    close $read_fh;
    print "$data";
} else {
    print "Error reading session file for session $sessid: $!";
}
[download]

And finally, I agree with roboticus that you should stick with passing the session id in the form rather than using globals. What happens if one user starts a session and forgets about it and then opens another browser window and trys to upload something else? They wind up reusing the first session... havock ensues.