my $blank_needed = 0;

open(DIFFR, "javaex.log");

while (<DIFFR>) {
   if (/at / || /xception/) {
      chomp;
      print LOGFILE "$_,\n";
      $blank_needed = 1;
   } else {
      print("\n") if $blank_needed;
      $blank_needed = 0;
   }
}
print("\n") if $blank_needed;