Well, tr appears to win if the 'M' is a common occurrence.
#! /usr/bin/perl -lw
sub cnt_a {
my ($count, $str) = (0, shift);
++$count while ($str =~ /(M+)/g);
$count;
}
sub cnt_b {
my ($count, $str) = (0, shift);
++$count while ($str =~ /M+/g);
$count;
}
sub cnt_c { int (() = shift =~ /M+/g) }
sub cnt_y { shift =~ y///csr =~ y/M// }
use Benchmark qw(cmpthese);
my @S = qw(I M O X);
# my @S = map chr, 0..255;
our $str = pack "(a)*", map $S[rand @S], 1..100000;
print for map $_->($str),
qw(CORE::length cnt_a cnt_b cnt_c cnt_y);
cmpthese -3, {
a => 'cnt_a($str)',
b => 'cnt_b($str)',
c => 'cnt_c($str)',
y => 'cnt_y($str)',
}