I'm working ona small CPAN module, File::Find::Match, which allows one to write rules for finding and processing files, sort of like File::Find but with more control over recursion into directories and such.

Currently, before calling each "action" (code that is executed when a file matches a pattern, basically) I set $_ to the file name. However, as I'm writing this in a hybrid functional/OO style, I found I dislike using the global.

So, now I'm going to be passing the filename as the first argument to the coderef. I thought this would be slower, and I wanted to know how much slower, so I bench marked it: 

#!/usr/bin/perl

use strict;
use warnings;


use Benchmark qw(cmpthese);


cmpthese(-60, {
        arg => sub {
            arg('foobar');
        },
        arg_shift => sub {
            arg_shift('foobar');
        },
        noarg => sub {
            $_ = 'foobar';
            noarg();
        },
        noarg2 => sub {
            $_ = 'foobar';
            noarg2();
        },
    });
sub arg       { length $_[0] }
sub arg_shift { length shift }
sub noarg     { length $_    }
sub noarg2    { length     }

[download]
The results of that (on Debian Sarge, perl 5.8.4 w/threads, on a 450mhz PII with 377MB of RAM) is: 
Rate     noarg    noarg2 arg_shift       arg
noarg     181644/s        --       -6%      -45%      -54%
noarg2    193848/s        7%        --      -41%      -50%
arg_shift 329166/s       81%       70%        --      -16%
arg       391348/s      115%      102%       19%        --

[download]

So it appears accessing $_[0] is faster than accessing $_. And even shift() is faster than using $_.

So now all that is left is the usability. Is it much more of a pain to write actions using $_[0] than $_?

I'm also curious why passing an argument is faster than accessing $_. :)

Update: I should mention, File::Find::Match is being used to create a Make replacement that works recursively over many trees. Like the ttree script from Template Toolkit.

In reply to $_ vs. argument passing by Dylan

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