sysopen
Opens a file using POSIX open(2) flags for precise mode control.
Unlike the high-level open, sysopen takes explicit POSIX mode
flags (from Fcntl) and an optional permissions mask. The flags
are passed directly to the underlying open(2) system call.
Arguments on the stack (via mark): filehandle target, filename, mode flags, and optional permissions (defaults to 0666, modified by umask).
Returns 1 on success (the filehandle is stored in the first argument) or undef on failure.
Synopsis
use Fcntl;
sysopen(my $fh, $filename, O_RDWR | O_CREAT, 0644);
sysopen(FH, $path, O_WRONLY | O_TRUNC);