From documentation:

The accept() method will return when a connection from a client is available. In a scalar context the returned value will be a reference to a object of the HTTP::Daemon::ClientConn::SSL class which is another IO::Socket::SSL subclass.

So if connection isn't SSL accept returns udef as HTTP::Daemon::ClientConn::SSL object wasn't created and your loop stops.

Update: this works:

while () {
    my $c = $d->accept or next;
    while ( my $r = $c->get_request ) {
        $c->send_file_response('./index.html');
    }
    $c->close;
    undef($c);
}
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