# The original subroutine

rot13sub "orig", sub {
  # as close as possible to source project's code
  join '', map { exists $rot{$_} ? $rot{$_} : $_ } split('',shift);
};

# Firstly, let's fill in the defaults in the hash table so that we 
# can use a hash slice

my %rotd = %rot;
exists($rotd{$_}) or $rotd{$_} = $_ for 
  map chr, 0 .. 255;

rot13sub "hashslice-s", sub {
  join '', @rotd{split('',shift)};
};