while (<DATA>) {
  /\./ ? s/(?<=\d)(?=(\d{3})+(?:\.))/,/g
       : s/(?<=\d)(?=(\d{3})+(?!\d))/,/g;
  print "$_";
}
__DATA__
123
1234
12345
123446
1234567
+12345678.12
-123456789.12345678

##</code><code>##

123
1,234
12,345
123,446
1,234,567
+12,345,678.12
-123,456,789.12345678