Okay, looks like my IO::Scalar suggestion is not going to work. I guess that leaves unpack(), substr(),split(), and the regex ?

Looks like unpack() is the clear winner on my machine:

#!/usr/bin/perl
use Benchmark;
my $string="X" x 102400;
timethese(100, {
             'split' => sub { 
                 for (split(//,$string)) {}; 
             },
             'unpack' => sub { 
                 for (unpack("C*",$string)) {}; 
             },
             'regex' => sub { 
                while($string =~ /./sg) {} 
             },
             'substr' => sub { 
               for(my $i=0;$i<length($string);$i++){
                     substr($string,$i,1);
               }
             },
});
[download]

Gives me:

$ perl foo
Benchmark: timing 100 iterations of regex, split, substr, unpack...
     regex: 44 wallclock secs (43.13 usr +  0.00 sys = 43.13 CPU)
     split: 49 wallclock secs (47.90 usr +  0.04 sys = 47.94 CPU)
    substr: 58 wallclock secs (55.70 usr +  0.00 sys = 55.70 CPU)
    unpack: 27 wallclock secs (26.48 usr +  0.00 sys = 26.48 CPU)

Update:Reposted results after correcting typo.