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

my $chars_to_change = join('', map quotemeta, keys %rot);

sub rot13dynre {
   local $_ = @_ ? $_[0] : $_;
   s/([$chars_to_change])/$rot{$1}/g;
   return $_;
}