I did a little, very basic, test, and I find the result quite interesting (and also confusing):

#!/usr/bin/perl

use Benchmark;

timethese(10000, {
        'foreachmy' => sub {
                my ($sum,$avg);
                foreach my $i (1..1000) {
                        $sum += $i;
                        $avg = $sum/$i;
                }
        },
        'foreachglobal' => sub {
                my ($sum,$avg,$i);
                foreach $i (1..1000) {
                        $sum += $i;
                        $avg = $sum/$i;
                }
        },
        'foreachnovar' => sub {
                my ($sum,$avg);
                foreach (1..1000) {
                        $sum += $_;
                        $avg = $sum/$_;
                }
        },
        'fornovar' => sub {
                my ($sum,$avg);

                for (1..1000) {
                        $sum += $_;
                        $avg = $sum/$_;
                }
        },
        'ctypeFor' => sub {
                my ($i,$sum,$avg);
                for ($i=1;$i<=1000;$i++) {
                        $sum += $i;
                        $avg = $sum/$i;
                }
        }
        }
);
[download]

Benchmark: timing 10000 iterations of ctypeFor, foreachglobal, foreach
+my, foreachnovar, fornovar...

  ctypeFor: 26 wallclock secs (26.27 usr +  0.00 sys = 26.27 CPU)

foreachglobal: 22 wallclock secs (22.50 usr +  0.00 sys = 22.50 CPU)

 foreachmy: 23 wallclock secs (22.42 usr +  0.00 sys = 22.42 CPU)

foreachnovar: 23 wallclock secs (23.33 usr +  0.00 sys = 23.33 CPU)

  fornovar: 24 wallclock secs (23.44 usr +  0.03 sys = 23.47 CPU)
[download]

While I'm not so sure about the little difference between the foreachlocal and foreachmy result, the construct foreach local $i (.....) definitely seems to be faster than foreach (......) ... or am I doing something terrible to $_ IN the foreach block?