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.

Here's another example:

use warnings;
use strict;

{
    package Tmp;
    use overload '""' => \&str, fallback => 1;

    sub c {
        bless [$_[1]], $_[0]
    }

    sub str {
        print "Hello, $_[0][0]\n";
        "." x rand 10
    }
}

my @strs = map Tmp->c($_), qw[foo bar baz];
my @str_lengths = map { length } @strs;
print "$_\n" for @str_lengths;
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As you can see, this time each call to length prints something to STDOUT and calls rand, which modifies global state. This is another reason why perl can't optimize such a loop in the general case: it's very hard to prove that @strs isn't tied or contains objects with overloaded stringification.