I don't see in your code why you would need a self-pipe+fork.

For reading from an asynchronous process, I've used a plain pipe-open quite successfully:

my $cmd = 'sdb shell hcidump |';
my $child = open( my $hci, $cmd )
    or die "Couldn't spawn [$cmd]: $!";

while (<$hci>) {
    ...
};

END {
    if( $child ) {
        kill 'KILL' => $child;
    };
    $child = 0;
};
[download]

Update: There was the pipe missing from the command line, whoops!