Perhaps the best way would be to reverse the number and then add in commas every three, and reverse back, such as:

#!/usr/bin/perl -w
use strict;

sub commafy {
        my ($num)=@_;

        $num=reverse $num;
        $num=~s/(\d{3})(?=\d)/$1,/g;
        $num=reverse $num;
}
      
my $example = commafy(123456789);

print "$example\n";
[download]

Picked up that little trick here, and it's rather useful.

=Blue
...you might be eaten by a grue...

UPDATE: The links Fastolfe gave lead to more complete answers then mine, which only deals with integers.