The problem is because I have two heavy processes for loading storable data inside the subrountine. After I moved those two out from the subrountine. The problem brothered me is gone. I guess that it is related to system memory swrap and memory allocation . I am sorry for not providing enough information earlier. Thanks a lot!

use Storable( retrieve, store)
my $string = getString($ARGV[]);
    ENDING MY TIMER

  sub getString{
      my ($file1, $file2) = @_;
      my @thisarray = @{retrieve($file1)}; # size ~ 20M
      my %thishash = %{retrieve($file2)}; # size ~ 5M     
      
      my $thisString;
      # here I am doing something 
      # to create a $thisString and its size is less than 100K 
     STARTING MY TIMER 
     return $thisString;
   }
[download]