my $wrap_len = 10;

my $buf = '';
LOOP:
for (;;) {
   while (length($buf) < $wrap_len+1) {
      my $rv = read(DATA, $buf, $wrap_len+1-length($buf), length($buf));
      die if not defined $rv;
      last LOOP if not $rv;
   }
   
   ($buf =~ s/^(.*)\n// || $buf =~ s/^(.{0,$wrap_len})//)
      and print("$1\n");
}

print($buf);

__DATA__
abcdefghijklm
ABCDEFGHIJKLM