use strict;
use warnings;

my @nos = qw{
   1
   12
   123
   1234
   12345
   123456
   1234567
   12345678
   123456789
   1234567890
   12345678901
   123456789012
   1234567890123
   12345678901234};

printf qq{%20s\n}, useSubstr($_) for @nos;

sub useSubstr
{
    my $number = shift;
    my $length = -( 1 + length $number );
    my $offset = -3;
    while ( $offset > $length )
    {
        substr $number, $offset, 0, q{,};
        $offset -= 4;
    }
    return $number;
}
[download]

produces

                   1
                  12
                ,123
               1,234
              12,345
             123,456
           1,234,567
          12,345,678
         123,456,789
        1234,567,890
      12,345,678,901
     123,456,789,012
    1234,567,890,123
   12345,678,901,234
[download]

As you can see, it breaks out of the while too soon as it has not kept up with the increasing length of the number as the commas go in. I agree that it would be nice to factor abs and length out of the loop but can't quite see how to achieve it.

Cheers,

JohnGG

Right, I didn't test very much.   :-) This works correctly and appears to be about 10% faster:

sub useSubstr { 
    my $number = shift;
    my $len = length $number;
    my $offset = 3;
    while ( $offset < $len ) {
        substr $number, -$offset , 0, q{,};
        $offset += 4;
        $len = length $number;
        }
    return $number;
    }
[download]

Nice ++. I don't know why I couldn't suss that one out myself. Perhaps I'd worn my brain out :(