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];
}