open my $in , "<", 'input.txt';
open my $out, ">", 'output.txt';

$/ = "^L"; # Set the end of page sequence

while (my $page = <$in>) # Get page
{
  if ($page =~ m/\nINVENTORY +(\d+-\d+)/) # Get the inventory number
  {
    my $inventorNumber = $1;

    # do your database lookup to get new date from inventory number
    my $date = doDatabaseLookup($inventoryNumber); 

    # Replace the existing date with the new date
    $page =~ s/(\nMY COMPANY PRODUCT #\n\d+ +)(\d{8})/$1$date/; 
  }
  print out $page;
}