Rate   wmap  count  wgrep wregex windex
wmap   5.42/s     --   -12%   -60%   -97%   -98%
count  6.13/s    13%     --   -54%   -97%   -98%
wgrep  13.5/s   148%   120%     --   -93%   -96%
wregex  198/s  3558%  3132%  1372%     --   -37%
windex  317/s  5741%  5062%  2251%    60%     --

##</code><code>##

use strict;
use warnings;

use Benchmark 'cmpthese';

srand( 0 );
my $s = join '', map chr(97+int(rand(26))), 1..100000;

cmpthese( -2,
          {
           windex   => \&windex,
           wregex   => \&wregex,
           wgrep    => \&wgrep,
           count    => \&count,
           wmap     => \&wmap,
          }
        );

sub windex {
  my @o;
  my $o = -1;
  while ( ( $o = index( $s, 'a', $o+1 )) > -1 ) { push @o, $o }
  return;
}

sub wregex {
  my @o;
  $s =~ m/a(?{ push @o, pos() - 1 })(?!)/;
  return;
}

sub wgrep {
  my @o = grep substr( $s, $_, 1 ) eq 'a', 0..length($s)-1;
  return;
}

sub count {
  my @o;
  my $count = 0;
  for ( split //, $s ) {
    push @o, $count if $_ eq 'a';
    ++$count;
  }
  return;
}

sub wmap {
  my $count = 0;
  my @o = map { $count++; /a/ ? $count - 1 : () } split //, $s;
  return;
}