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;
   }