I thought that too, but it isn't faster:

use warnings;
use strict;

use Benchmark qw(:all);

open my $fh, '<', $ARGV[0] or die $!;

cmpthese(100, {
                all => '_all',
                while => '_while',
         });

sub _all {
    seek $fh, 0, 0;
    my $count = 0;
    while (<$fh>){
        $count += () = /(M+)/g;
    }
#    print "all: $count\n";
}
sub _while {
    seek $fh, 0, 0;
    my $count=0;
    while(<$fh>){
      while($_=~/(M+)/g){
          $count++;
      }
    }
#    print "while: $count\n";
}

__END__

        Rate   all while
all   11.5/s    --  -25%
while 15.2/s   33%    --
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