my $buf    = '';
my $offset = 0;

DATA:
for (;;) { # For each record
   my $buf    = '';
   my $offset = 0;
   my $rec;

   do { # Until we have a record.
      my $len = sysread(STDIN, $buf, 1024, $offset);

      die("Unable to read from XXX: $!\n")
         if not defined $len;

      die("Unable to read from XXX: Premature end of file\n")
         if not $len and $offset;

      last DATA if not $len;

      my $pos = index($buf, "\x0D\x0A", $offset);
      $offset += $len;
   } while $pos < 0;

   $rec = substr($buf, 0, $pos+2);
   $buf = substr($buf, $pos+2);

   ... process $rec ...
}