if (@ARGV) { # something's in @ARGV so we'll use it
  use open ':raw'; # implicit binmode on each open
  open(ARGV, shift @ARGV); # Yeah, it's the two-arg version,
                           # because that's what perl does with <>
                           # see perlopentut
} else {     # nothing's in @ARGV so we'll use STDIN
  use open ':raw'; # implicit binmode on each open
  open(ARGV, '-');
}
# ... now go do all that sysread stuff.