yet another way to do it:

use strict;
use warnings;
my $string= "012345678901234567890123456789";
$string = sprintf "%5s-%5s-%5s<br>%5s-%5s-%5s",$string=~/(\d{5})/g;
print $string,$/;
[download]

update: I saw Tie::Cycle earlier in this node so you could also do it this way:

use strict;
use warnings;
use Tie::Cycle; 

tie my $cycle, 'Tie::Cycle', [ qw( -  - <br> ) ];
my $string= "012345678901234567890123456789";
$string =~ s/(\d{5})(?!\b)/$1$cycle/g;
print $string;
[download]

-enlil