use HTTP::Request::Common;

sub my_post_file {
    my $bufsize = 10_240_000;
    local *HTTP::Request::Common::form_data = sub {
        ...
	my $buflength = length $buf;
	my $n = read($fh, $buf, $bufsize, $buflength);
        if ($n) {
	    $buflength += $n;
	    unshift(@parts, ["", $fh]);
	}
        ...
    };