sub POSIX_FIONBIO () { (0x80000000 | (4<<16) | (ord('f')<<8) | 126) }  # 0x8004667E

ioctl($sock, POSIX_FIONBIO, pack("I", 1))
or $! == 0  # **
or die(...);

# ** - ioctl is returning undef,
#      yet both $! and $^E are 0.
#      I don't know what this means.