# Now let's use an array slice instead of a hash

my @rot = do {
  my $c;
  map { $c = chr($_); exists $rot{$c} ? ord($rot{$c}) : $_ } 0 .. 255;
};

rot13sub "arrayslice", sub {
  pack "C*", @rot[unpack("C*", $_[0])];
};

# Now let's try some solutions with s, as demerphq has suggested. 
# The first one has to know the exact character range to be replaced.

rot13sub "gsub-class", sub {
  (my $s = $_[0]) =~ s/([a-zA-Z])/$rot{$1}/g;
  $s;
};

rot13sub "gsub-dot", sub {
  (my $s = $_[0]) =~ s/(.)/$rotd{$1}/gs;
  $s;
};

# Now your tr equivalent

rot13sub "tr", sub {
  (my $s = shift) =~ tr/a-zA-Z/n-za-mN-ZA-M/;
  $s;
};