Rate  Split   Molt    FAQ Cdarke PrefUN Substr
Split  8.00/s     --   -15%   -31%   -39%   -53%   -74%
Molt   9.36/s    17%     --   -19%   -29%   -45%   -70%
FAQ    11.5/s    44%    23%     --   -12%   -32%   -63%
Cdarke 13.2/s    64%    41%    14%     --   -22%   -58%
PrefUN 16.9/s   111%    80%    47%    28%     --   -46%
Substr 31.3/s   291%   234%   171%   138%    85%     --

##</code><code>##

use strict;
use warnings;

use Benchmark qw(cmpthese);

my @numbers;
push @numbers, int rand 1e9 for 1 .. 1000;

my $rcUseCdarke = sub
   {
       my $res = useCdarke($_) for @numbers;
   };

my $rcUseFAQ = sub
   {
       my $res = useFAQ($_) for @numbers;
   };

my $rcUseMolt = sub
   {
       my $res = useMolt($_) for @numbers;
   };

my $rcUsePrefUN = sub
   {
       my $res = usePrefUN($_) for @numbers;
   };

my $rcUseJwkrahn = sub
   {
       my $res = useJwkrahn($_) for @numbers;
   };

my $rcUseSplit = sub
   {
       my $res = useSplit($_) for @numbers;
   };

my $rcUseSubstr = sub
   {
       my $res = useSubstr($_) for @numbers;
   };

cmpthese (50,
   {
      Cdarke => $rcUseCdarke,
      FAQ => $rcUseFAQ,
      Jwkrahn => $rcUseJwkrahn,
      Molt => $rcUseMolt,
      PrefUN => $rcUsePrefUN,
      Split => $rcUseSplit,
      Substr => $rcUseSubstr
   });

sub useCdarke
{
    my $number = reverse shift;
    $number =~ s/(\d{3})/$1,/g;
    $number = reverse $number;
    return $number;
}

sub useFAQ
{
    my $number = reverse shift;
    $number =~
       s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
    return $number;
}

sub useJwkrahn
{
    return scalar reverse join ',', unpack '(a3)*', reverse shift;
}

sub useMolt
{
    my $number = shift;
    while ($number =~ s/(\d)(\d\d\d)(?!\d)/$1,$2/) {};
    return $number;
}

sub usePrefUN
{
    my $num = reverse shift;
    my @threes = $num =~ /(.{1,3})/g;
    return scalar reverse join ',', @threes;
}

sub useSplit
{
    my $ct = 0;
    my $len = length $_[0];
    return join q{},
       reverse map
       {
           ++ $ct % 3
              ? $_
              : $ct == $len
                 ? $_
                 : ($_, q{,})
       }
       reverse split m{}, $_[0];
}

sub useSubstr
{
    my $number = shift;
    my $offset = -3;
    while (abs $offset < length $number)
    {
        substr $number, $offset , 0, q{,};
        $offset -= 4;
    }
    return $number;
}

##</code><code>##

          Rate   Split    Molt     FAQ  Cdarke  PrefUN Jwkrahn  Substr
Split   7.85/s      --    -16%    -31%    -41%    -54%    -70%    -75%
Molt    9.31/s     19%      --    -19%    -31%    -45%    -64%    -70%
FAQ     11.4/s     46%     23%      --    -15%    -32%    -56%    -63%
Cdarke  13.4/s     71%     44%     17%      --    -21%    -48%    -57%
PrefUN  16.9/s    115%     81%     48%     26%      --    -35%    -45%
Jwkrahn 25.9/s    230%    178%    126%     93%     53%      --    -16%
Substr  30.9/s    293%    231%    170%    130%     83%     19%      --