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

[download]
Which will give the following result: 
123
1,234
12,345
123,446
1,234,567
+12,345,678.12
-123,456,789.12345678

[download]
Which is exactly what you wanted.

In reply to Re: Comma-fy floats with (+/-)look(aheads/behinds) by Roger
in thread Comma-fy floats with (+/-)look(aheads/behinds) by Anonymous Monk

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