The problem was solved by closing the file GPsmk4 (see below). Leaving this filehandle open caused all my problems! Thanks for your suggestions ! Regards, Stacy.

open(CLOCK,"$gps2_log") or die "$gps2_log not available: $!\n";
  open(GPSmk4,">>/tmp/gpsmk4.dat");

  while($line = <CLOCK>) {
    next if $line =~ /last/;
    ($month,$day,$time) = ( split(/\s+/,$line) )[0..2];
    ($hour,$min,$sec) = split(':',$time);

    $month = sprintf "%02d", $months{$month};
    $day   = sprintf "%02d", $day;
    
    $data3 = ( split(/\s+/,$line) )[6];
       
last if(compare_dates([$month,$day,$hour,$min,$sec],$edate_cursor)>0);
if (compare_dates([$month,$day,$hour,$min,$sec],$sdate_cursor)>0);
      {
        if( $data3 > $plotymax ) { $plotymax = $data3; }
        if( $data3 < $plotymin ) { $plotymin = $data3; }
        print GPSmk4 "$month-$day $time $data3\n";
      }
    # update the date cursor
    $date_cursor = [$month,$day,$hour,$min,$sec];
    
 +close(GPSmk4);

  } #end of while
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