#! /usr/bin/perl
use warnings;
use strict;

use Benchmark qw{ cmpthese };

sub outer {
    my $s = 0;
    my $x;
    for $x (1 .. 5) {
        $s += $x;
    }
    $s
}

sub inner {
    my $s = 0;
    for my $x (1 .. 5) {
        $s += $x;
    }
    $s
}

outer() == inner() or die 'Error in implementation';

cmpthese(-2, {
    outer => 'outer()',
    inner => 'inner()',
});
[download]

The result on my machine shows inner is about 6% faster. Such a small difference is insignificant and usually has no real impact on real performance.

Why is that? Remember that for localises its variable when it's not lexical, i.e. it has to store its previous value before entering the loop and restore it at the loop's end. It seems to take a bit more time than just creating a fresh new lexical variable.