I don't know how well the optimizer works, but if you really need the last milklisecond, don't get the fields out, but keep working with the reference. Also drop the chomp. You're not working with C<$_> anymore.

DOIT: while (my $it = $csv->getline ($io)) {
          my (%rec, %HoH, $p);
          # chomp;             # No need to chomp anymore!
          $t_counter++;
          # my @fields = @$it; # Don't make a copy
          if ($it->[0] =~ /^STOP/) { # use the reference
              :
              }
          elsif ($it->[0] ...
[download]