Heh, add a couple more decimal places and it will fail (unless you consider commas after the decimal point to be allowable):

#!/usr/bin/perl -w
use strict;
my $number='12345.67890'; # with commas, should be: 12,345.67890 
print "N=$number\n";
$number =~ s/(\d)(?=(\d{3})+(\D|$))/$1\,/g;
print "N=$number\n";
[download]

Output:

N=12345.67890
N=12,345.67,890
[download]

Looks funky to me.