$_ = "mypic.jpg.jpg";
if (/^(.*)(\..*)/s) { 
        my($p, $s) = ($1, $2); 
        $p=~y/./_/; $_ = $p.$s; 
}
print $_, $/;

##</code><code>##

$_ = "mypic.jpg.jpg";
/^(.*)\./s and substr($_, 0, $+[1])=~s/\./_/g;
print $_, $/;