return scalar reverse $num;
[download]

sub format_cc_num {
    my ($num, $spacer) = @_;
    scalar reverse join $spacer, (reverse $num) =~ /(....)/g;
}
[download]

Makeshifts last the longest.

Actually, we both were buggy. Using your code and my code (with the appropriate bugfix), we have:

#!/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
[download]

Your code only works for card numbers that are a multiple of 4. Most department store cards are 11 digits and some, like AmEx, are moving to 17 or 19 digits. (I used to work for MC ... we had a similar function that I had to maintain in the app I worked on. It was written horribly with substr and the like.)

------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.

Oops. I was going to use unpack at first and didn't think about that when going for the pattern instead. The fix is obvious, though.

sub format_cc {
   my ($n, $s) = @_;
   scalar reverse join $s, (reverse $n) =~ /(.{1,4})/g;
}
[download]

Makeshifts last the longest.