There might be an easier way but you could do this:

open (DIFFR, "/tmp/log.diff");
while (<DIFFR>)
{
  $line=$_;
  chomp($line);
  if ($line =~ /ERROR/)
  {
    print CSVFILE "$oldline\n"; #look below if stat.
    print CSVFILE "$line,";
  }
  $oldline = $line; #sets a variable with the last line
}
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