#!/usr/bin/perl
 
use strict;
use warnings;
 
my $x = "12345678";
my $y = "123456789";
 
sub dragonchild
{
   my ($n, $s) = @_;
 
   $n = reverse $n;
   # Added negative lookahead
   $n =~ s/(.{4})(?!$)/${1}${s}/g;
   scalar reverse $n;
}
 
sub aristotle
{
   my ($n, $s) = @_;
   scalar reverse join $s, (reverse $n) =~ /(....)/g;
}
 
foreach my $v ($x, $y)
{
   print dragonchild($v, '-'), $/;
   print aristotle($v, '-'), $/;
}

------

1234-5678
1234-5678
1-2345-6789
2345-6789