#!/usr/bin/perl
use warnings;
use strict;

use Test::More;
use Benchmark qw{ cmpthese };

sub regex {
    my ($string, $find, $replace) = @_;
    $string =~ s/\Q$find/$replace/g;
    $string
}

sub substring {
    my ($string, $find, $replace) = @_;
    my $length = length $find;
    my $pos = 0;
    while (-1 != ( $pos = index $string, $find, $pos )) {
        substr $string, $pos, $length, $replace;
    }
    $string
}

sub array {
    my ($string, $find, $replace) = @_;
    join $replace, split /\Q$find/, $string, -1
}

my @args = ('12a345a678a90', 'a', '||');
is regex(@args), '12||345||678||90', 'manual';

for my $args (\@args,
              [ '123456789abcdefghijkl' x 1000,
                '123456789abcdefghijkl' x 21,
                'ABCDEFGH'
              ]) {
    is regex(@$args), substring(@$args), 'regex - substring';
    is substring(@$args), array(@$args), 'substring - array';

    cmpthese(-3,
             { regex     => sub { regex(@$args) },
               substring => sub { substring(@$args) },
               array     => sub { array(@$args) },
           });
}
done_testing(5);
[download]

Output on my machine (Perl 5.20.1, Linux on Intel i3):

ok 1 - manual
ok 2 - regex - substring
ok 3 - substring - array
              Rate     regex substring     array
regex     313491/s        --      -21%      -36%
substring 398041/s       27%        --      -19%
array     488419/s       56%       23%        --
ok 4 - regex - substring
ok 5 - substring - array
             Rate substring     array     regex
substring 18505/s        --      -15%      -15%
array     21765/s       18%        --       -0%
regex     21765/s       18%        0%        --
1..5
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

I wrote a little program that runs a million string replace operations using the three methods that have been suggested here so far, and it displays how many seconds it takes to run each replace operation. (I am using tinyperl 5.8 on Windows XP Pro SP2. And I am a beginner Perl programmer.)

As it turns out, if the length of the resulting string is longer than the original after the replacement, then it takes longer to do the replacement. Btw, the regexp has been the fastest method. The split/join method was average. And the substr method was the slowest! It was so slow that I had to abort the program after it replaced 1 million "HelloWorld" strings to "XXWorld." It took more than an hour! Anyway, here is the code: