my %rot;
@rot{'A'..'M'} = ('N'..'Z');
@rot{'N'..'Z'} = ('A'..'M');
@rot{'a'..'m'} = ('n'..'z');
@rot{'n'..'z'} = ('a'..'m');

*rot13builttr = eval 'sub {
   local $_ = @_ ? $_[0] : $_;
   tr{' . join('', map quotemeta, keys   %rot) . '}
     {' . join('', map quotemeta, values %rot) . '};
   return $_;
}';