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    my $new_string = testing_string($string); 

...
        $content =~ s/a/b/sg;
        return $content; #string copy, expensive
    }

   # do stuff here
    my %test_var;
...
        # do some stuff here
        $test_var{$string} =~ s/a/b/sg;  
    }

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

...
        # do some stuff
        $$contentref =~ s/a/b/sg;
   }