Quite inconclusive. Considering the what the optimizer does, it highly depends on your data whether index() or a regex is faster. It also depends whether there is a match, where the match is (if any), and whether the string has been studied. Here's some more data: 
#!/usr/bin/perl

use strict;
use warnings;

use Benchmark 'cmpthese';

our $string  = "abcd" x 1000;
    $string .= "e";
    $string .= "abcd" x 1000;
our $study   = $string;
our $pass    = "abcde";
our $fail1   = "foo12";
our $fail2   = "abdce";

study $study;

cmpthese(-1, {
    index_pass => 'index($string, $pass)',
    regex_pass => '$string =~ /$pass/',
    study_pass => '$study  =~ /$pass/',
});
print ("\n\n");

cmpthese(-1, {
    index_fail1 => 'index($string, $fail1)',
    index_fail2 => 'index($string, $fail2)',
    regex_fail1 => '$string =~ /$fail1/',
    regex_fail2 => '$string =~ /$fail2/',
    study_fail1 => '$study  =~ /$fail1/',
    study_fail2 => '$study  =~ /$fail2/',
});

__END__
               Rate index_pass study_pass regex_pass
index_pass  38331/s         --        -6%       -69%
study_pass  40960/s         7%         --       -67%
regex_pass 125463/s       227%       206%         --


                 Rate index_fail2 study_fail2 index_fail1 regex_fail2 
+regex_fail1 study_fail1
index_fail2   27306/s          --         -0%        -48%        -56% 
+       -64%        -99%
study_fail2   27307/s          0%          --        -48%        -56% 
+       -64%        -99%
index_fail1   52608/s         93%         93%          --        -15% 
+       -31%        -98%
regex_fail2   61837/s        126%        126%         18%          -- 
+       -19%        -98%
regex_fail1   75918/s        178%        178%         44%         23% 
+         --        -98%
study_fail1 3412032/s      12396%      12395%       6386%       5418% 
+      4394%          --

[download]
Note that with this data, index is slower than a regex. The fastness of 'study_fail1' is explained by the fact that the string we are looking for, 'foo12', contains letters not present in the string - and since a studied string has a histogram attacked of its letter frequencies, no searching needs to be performed at all.

In reply to Re^6: "advanced" Perl functions and maintainability by Anonymous Monk
in thread "advanced" Perl functions and maintainability by geektron

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