The problem is that Perl provides no way to guarantee that a given piece of code has no side effects.
Indeed. And in Perl, this is very, very hard. How hard? Well, your own example fails the test. Even the map {length} @array may cause a side-effect. 
    use Devel::Peek;
    my @arr = ("foo", 3);
    Dump($arr[-1]);
    my @bogus = map {length} @arr;
    Dump($arr[-1]);
    __END__
    SV = IV(0x8192bcc) at 0x8184234
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 3
    SV = PVIV(0x8184888) at 0x8184234
      REFCNT = 1
      FLAGS = (IOK,POK,pIOK,pPOK)
      IV = 3
      PV = 0x8189cf8 "3"\0
      CUR = 1
      LEN = 2

[download]
Whoopsie. One of the elements of @arr changed, just by looking at it!

This is the same reason why threads in Perl take quite a lot of overhead, and that by default variables are copied (and not shared) between threads.

In reply to Re: RFC: Implicit Parallelization Pragma by Anonymous Monk
in thread RFC: Implicit Parallelization Pragma by hardburn

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