Many of us know that foreach (@array) {...} is optimized - no extra copying, just some iterations over array. But it was not obvious for me whether or not more complex constructs are optimized this way.
In such cases they better be written as: for ($i=0;$i<=$#array;$i++) {...}

So I wrote simple benchmark program and found out that optimizations which perl performs are quite good:

use strict;
use Benchmark;

my ($x,$i);

my @f = ('a'..'zz');
my $f = \@f;

sub first_one {
  for (@f) {$x++}
}
sub second_one {
  for (@$f) {$x++}
}
sub third_one {
  for ('a'..'zz') {$x++}
}
sub fourth_one {
  for ($i=0;$i<=$#f;$i++) {$x++}
}

timethese(10_000, {
    'first' => \&first_one,
    'second' => \&second_one,
    'third' => \&third_one,
    'fourth' => \&fourth_one,
});
[download]

Benchmark: timing 10000 iterations of first, fourth, second, third...
     first:  6 wallclock secs ( 6.02 usr +  0.00 sys =  6.02 CPU) @ 16
+61.68/s (n=10000)
    fourth: 12 wallclock secs (11.98 usr +  0.00 sys = 11.98 CPU) @ 83
+4.86/s (n=10000)
    second:  7 wallclock secs ( 6.00 usr +  0.00 sys =  6.00 CPU) @ 16
+67.22/s (n=10000)
     third:  9 wallclock secs ( 9.57 usr +  0.00 sys =  9.57 CPU) @ 10
+44.50/s (n=10000)
[download]