sub has_dup_letters {
  local $_ = shift;
  tr/a-zA-Z//cd;
  tr/A-Z/a-z/;
  my %dups;
  scalar grep $dups{$_}++ for split //;
}

##</code><code>##

scalar grep $dups{$_}++ for split //;

##</code><code>##

scalar grep $dups{$_}++,  split //;