Whenever I see "format" and "number", I think of... Number::Format :)

use warnings;
use strict;
use Number::Format qw(format_picture);

print format_picture(4123456789, '##,##,##,##,##,##,###'), "\n";

__END__

        4,12,34,56,789
[download]

You could consider this as an alternative to regular expressions. One disadvantage is that format_picture forces you to know the upper bound on your number's length.