sub commify { 
        local $_  = shift; 
        1 while s/^(-?\d+)(\d{3})/$1,$2/; 
        return $_; 
    } 
  
    $n = 23659019423.2331; 
    print "GOT: ", commify($n), "\n"; 
  
    GOT: 23,659,019,423.2331

    s/^(-?\d+)(\d{3})/$1,$2/g;

    # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca> 
    sub commify { 
       my $input = shift; 
        $input = reverse $input; 
        $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g; 
        return reverse $input; 
    }