No need for a new perl and the /r option either. This works fine:

#! perl -slw
use strict;
use Time::HiRes qw[ time ];

my $start = time;
my $count=0;
while( <DATA> ) {
#    $count += ()= m{M+}g;
    tr/M/M/s;
    $count += tr/M/M/;
}
print $count."\n";

printf "Took %9f secs\n", time() - $start;
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