while ($_ ne '') {
   $letters{lc chop}++;
}

##</code><code>##

$letters{$_}++ for split//, lc;


##</code><code>##

$letters{lc $_}++ for split//;


##</code><code>##

while ($_ ne '') {
   $letters{lc substr($_,-1)}++;
   substr($_, -1) = "";
}