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;
}