This is taken from perlman:perlsub:

When to Still Use local()

Despite the existence of my(), there are still three places where the local() operator still shines. In fact, in these three places, you must use local instead of my.

1. You need to give a global variable a temporary value, especially $_.

   The global variables, like @ARGV or the punctuation variables, must be localized with local(). This block reads in /etc/motd, and splits it up into chunks separated by lines of equal signs, which are placed in @Fields.

       {
           local @ARGV = ("/etc/motd");
           local $/ = undef;
           local $_ = <>;  
           @Fields = split /^\s*=+\s*$/;
       } 
   

   In particular, it's important to localize $_ in any routine that assigns to it. Look out for implicit assignments in while conditionals.

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Basically, you're asking benchmark to do one of two things. Either, it has to reset all the global variables to the states they were at just before 'timethese' was called. Alternatively, the benchmarked code could be examined by 'timethese' and determine which global symbols your code uses and only reset those.

I think both of these are silly and horrible, to put it bluntly (sorry :) ). If you want to reset a variable you should do it yourself. And notice that you are warned in the documentation that you should local any global variable that needs a temporary value. Of course, you can also just reset @_ or $_ with:
@_ = ();
Which (can) be faster than:
local @_;
However, you should note that the local call will keep memory usage down. Otherwise Perl doesn't seem to flush @_ = () memory in the same way.

Anyway, this feature of not reseting your states is surely not a "side effect". Having Benchmark reset the state of global variables (think about filehandles) as an option or otherwise would definately be an interesting endeavor. However, I do not think it would be worthwhile, nor possible to implement cleanly or quickly.

Seriously, how hard is it to put your own initialization code in? And also does it not make sense that you would need to reset your conditions yourself?? Lastly, the documentation (perl's) states you should local global variables that need temporary values, and Benchmark's documentation says nothing about reseting variables for you either.

But cheers anyway, :)
Gryn