my $i;
while (sysread FH, $digit, 1)
# same as: while (sysread(FH, $digit, 1) != 0)
# (well, not quite, but close enough here.)
{
  ++$i; # keep track of which digit we're looking at.
  # deal with the $digit you just got.
  printf "%04d %d\n", $i, $digit;
}