my $new_string = testing_string($string); 

    sub testing_string {
        my $content = $_[0]; #string copy, expensive
        # do some stuff
        $content =~ s/a/b/sg;
        return $content; #string copy, expensive
    }

##</code><code>##

   # do stuff here
    my %test_var;
    $test_var{$string} = $string; # double!! string copy and hash 
                     # sum calculation, expensive. And why   
                     # this senseless copying into a hash? a scalar 
                     # variable (i.e $string) is global too
    testing_string();
    $string = $test_var{$string}; # string copy and hash sum calculation of $string

    sub testing_string_2 {
        # do some stuff here
        $test_var{$string} =~ s/a/b/sg;  
    }

##</code><code>##

   testing_string(\$string); #send a reference to $string instead of $string

    sub testing_string {
        my $contentref = $_[0];
        # do some stuff
        $$contentref =~ s/a/b/sg;
   }