my $fil_count = 0;
my $delim = 'XXFFDDF';
open IN, 'in.txt' or die "Can't open in.txt: $!\n";
open OUT, '> out0.txt' or die "Can't write to out0.txt: $!\n";
while (<IN>)
{
  if (/^(.*?)$delim(.*)$/) {
    print OUT $1 if $1;
    close OUT;
    $fil_count++;
    open OUT, '> out' . $fil_count . '.txt' or die "Can't write to out${fil_count}.txt: $!\n";
    print OUT $2 if $2;
  }
  else {
    print OUT $_;
  }
}
close IN;