If all you want to do is check the presence of M using tr/// then it is simple:

#!/usr/bin/env perl 
use strict;
use warnings;

my $s='IIIIIIIIIIIIIIIIIIIIIIIIIIOOOOOSSSSS';

print "$s: " . ($s =~ tr/M/M/ ? "case1\n" : "case2\n");

$s .= 'M';

print "$s: " . ($s =~ tr/M/M/ ? "case1\n" : "case2\n");
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Or, if you don't care about tr///:

if ($s =~ /M/) {
    print "case1\n";
} else {
    print "case2\n";
}
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tr should be faster, though.

tr/M// is sufficient.

tr/M// is sufficient

Indeed it is ... as any golfer would know. :) Speaking of which, note that y is an oddball synonym for tr -- added, not for golfers (because code golf didn't exist back then), but to entice diehard sed devotees to the new frontier of Perl.

I have to give a further honourable mention to y///c -- aka Abigail's length horror -- because it's one character shorter than the prosaic length.

Finally, note that Perl's flexibility of being able to choose your own delimiter has made tr a favourite of obfuscators (and bored golfers) ... as I remember from this amusing expression:

($.|y|||c|y|a|||y|e|||y|i|||y|o|||y|u|||y|y||)||print
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employed in a (non-winning) entry in the pioneering Get Even golf game of 2002.