in reply to Faster way to do this?

my $string = 'IIIIIIIIIIIMMMMMMMMMMMMOOOOOOOOOOOOMMMMMMMMMIIIIIIIII';
my $count = () = $string =~ /M/g;
print $count;

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You can also use the tr operator.

my $count = $string =~ tr/M//;

In Section Seekers of Perl Wisdom