Hi. Three ways to do it.

One, slurp the file and parse it using subscripts. So when $line[$i] matches, u can pull $line[$i - 1] to get the other value.

Two, save the current line value into a variable which will be available on the next iteration of the loop.

while ($line = <FILE>) {
   if ($line is good) {
      dosomething($line, $lastline);
   }
   $lastline = $line;
}
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Three, use Tie::File.