in reply to function to count pending packets in TCP socket
I would continue to write data on my TCP socket until the send window will be 0 and eventually I may lose the data.
That is not how TCP works: once the TCP sending buffer becomes full, the kernel would block any write operation until data is transmitted to the other side leaving some buffer space free again. No data is lost.
If you don't want to block, you would have to use select or some of the modules wrapping it (or similar syscalls). For complex cases, using an event-driven programing framework such as AnyEvent, POE, Mojo::IOLoop, IO::Async, etc. may help too.
Update: I forget to say that no data is lost in TCP operations, but you have to check the result of syswrite which is the number of bytes written. It is not guaranteed that syswrite would write all the data passed, so you will have to call it repeatly, adjusting the offset until all the data is sent (well, actually, buffered by the kernel!).