use strict;
use warnings;

printf qq{%20s\n}, comma3($_)
   for (12, 1234, 1234567, 1234567890);

sub comma3
{
    my $ct = 0;
    join q{},
       reverse map {++ $ct % 3 ? $_ : ($_, q{,})}
       reverse split m{}, $_[0];
}

##</code><code>##

                  12
               1,234
           1,234,567
       1,234,567,890

##</code><code>##

sub comma3
{
    my $ct = 0;
    my $len = length $_[0];
    return join q{},
       reverse map
       {
           ++ $ct % 3
              ? $_
              : $ct == $len
                 ? $_
                 : ($_, q{,})
       }
       reverse split m{}, $_[0];
}