I could be wrong, but I think you're stuck. From the browser's perspective, it's sending some large file as part of a request to the server. For it to display anything, it has to be finished sending that request. If you're trying to read what it's sending and give it something to show, I don't see how that's going to work. With JavaScript, you can have a display going at the same time, but there isn't any "at the same time" in a single HTTP request.

As an aside, I usually see a fork done this way:

my $pid = fork();
die "Can't fork: $!\n" if ! defined $pid;
if ( $pid ) {
    # parent
}
else {
    # child
    exit;
}
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