While talking to revdiablo about the details behind the behavior of $_ vs. argument passing, I threw together this example of where things in Perl aren't always well-understood.

In C, the following code: 

// hypothetical struct x
if (flag) {
  x->y->z->a->b[5]->q = 8;
  x->y->z->a->b[5]->r = 9;
}

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gets compiled very efficiently with optimizations turned on, due to a standard optimization called 'common subexpression elimination' (heretofore referred to as 'CSE'). In effect, the compiler does this: 
if (flag) {
  t = x->y->z->a->b[5];
  t->q = 8;
  t->r = 9;
}

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Thus, the compiler only has to perform all these dereferences *once*. Unfortunately, unlike C, Perl has to contend with something a little crazier: Tied hashes and arrays. The nature of tied hashes and arrays makes certain things impossible, including CSE. To illustrate my point, I created a benchmark: 
#!/usr/bin/perl

use strict;
use warnings;

use Time::HiRes;
use Benchmark qw(cmpthese :hireswallclock);

my @bogus;

$bogus[1]{foo}[2]{bar}[3]{baz} = 5;

$bogus[1]{foo}[2]{bar}[3]{quux} = 6;

our @bogus2;

$bogus2[1]{foo}[2]{bar}[3]{baz} = 5;

$bogus2[1]{foo}[2]{bar}[3]{quux} = 6;

cmpthese(1_000_000, {
        without_common => sub {
           my $prod = $bogus[1]{foo}[2]{bar}[3]{baz} *
                      $bogus[1]{foo}[2]{bar}[3]{quux};
        },

        with_common => sub {
           my $common = $bogus[1]{foo}[2]{bar}[3];
           my $prod = $common->{baz} * $common->{quux};
        },

        without_common_g => sub {
           my $prod = $bogus2[1]{foo}[2]{bar}[3]{baz} *
                      $bogus2[1]{foo}[2]{bar}[3]{quux};
        },

        with_common_g => sub {
           my $common = $bogus2[1]{foo}[2]{bar}[3];
           my $prod = $common->{baz} * $common->{quux};
        },
    });

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The results on my machine are as such: 
                     Rate without_common_g without_common with_common_
+g with_common
without_common_g 341880/s               --            -4%          -20
+%        -23%
without_common   357910/s               5%             --          -16
+%        -19%
with_common_g    428449/s              25%            20%            -
+-         -3%
with_common      443853/s              30%            24%            4
+%          --

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Note carefully that with_common runs 24% faster than without_common. That's because, even with the extra variable, the three array index and two hash key lookups saved in with_common more than compensate for the extra variable assignment.

Also note the 4-5% difference between with_common/without_common and their _g equivalents. This is *entirely* due to the extra hash lookup required to get the address of the global 'bogus2'. This hash lookup is ALWAYS required due to dynamic scoping requirements.

Morals of the story:


Update: Realized how long this node is, added readmore.
--Stevie-O 
$"=$,,$_=q>|\p4<6 8p<M/_|<('=>
.q>.<4-KI<l|2$<6%s!<qn#F<>;$,
.=pack'N*',"@{[unpack'C*',$_]
}"for split/</;$_=$,,y[A-Z a-z]
         {}cd;print lc

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In reply to Perl and common subexpressions by Stevie-O

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