Here are a few alternatives assuming you cannot pre-split for your purpose.

My favorite hack is chop if I need speed and order does matter. It's about 4x faster than split and substantially faster than substr even if you have to reverse and copy for your needs. Better if you can avoid doing both, which you frequently can.

#! perl -sw
use 5.010;
use strict;
use Benchmark qw[ cmpthese ];

our $LEN ||= 100;
our $string = 'A' x $LEN;
our @chars = split//, $string;

cmpthese -3, {
    substr => q[
        our $string;
        for (  0 .. length $string ) {
            my $c = substr $string, $_, 1;
        }
    ],
    pre_split => q[
        our @chars;
        for my $c ( @chars ) {
            ;
        }
    ],
    split => q[
        our $string;
        my @chars = split //, $string;
        for my $c ( @chars ) {

        }
    ],
    unpack => q[
        our $string;
        for ( unpack 'C*', $string ) {
            my $c = chr;
        }
    ],
    chop => q[
        our $string;
        my $copy = $string;
        while( my $c = chop $copy ) {
            ;
        }
    ],
    rev_chop => q[
        our $string;
        my $copy = reverse $string;
        while( my $c = chop $copy ) {
            ;
        }
    ],
};

__END__
C:\test>byChar.pl
              Rate     split    unpack    substr  rev_chop      chop p
+re_split
split      11018/s        --      -74%      -74%      -81%      -82%  
+    -94%
unpack     42513/s      286%        --       -0%      -26%      -30%  
+    -75%
substr     42526/s      286%        0%        --      -26%      -30%  
+    -75%
rev_chop   57794/s      425%       36%       36%        --       -5%  
+    -66%
chop       61056/s      454%       44%       44%        6%        --  
+    -64%
pre_split 171734/s     1459%      304%      304%      197%      181%  
+      --
[download]