The first buffered read <$h> grabs all five of the first sent lines, and leaves the handle empty, thus the delay
for the second read. It's not until the 6th line comes in that the program can get to the second read.
The usual advice is to not mix buffered and un-buffered I/O, and this code is the perfect example of why.
IO::Select operates on unbuffered data while readline is buffered.
That's why I used sysread (which is unbuffered) rather than readline.
NOTE: Using sysread may get you more than one line at a time (or even partial lines) which is why I added the loop that extracts and deletes one while line at a time.