$s='III...MMMMMOOOO....MMMIIII';
print $s."\n";
%neg = ( O => "I", I => "O" );

$s=~s/([IO])(M*)(\.+)/ $1.$2.( $neg{$1} x length($3) ) /eg;
$s=~s/(\.+)(?=M*([IO]))/ $neg{$2} x length($1) /eg;

print $s;print "\n";
<code>
It prints:<br>
<code>
III...MMMMMOOOO....MMMIIII
IIIOOOMMMMMOOOOIIIIMMMIIII