Here's another way to comma'ify your numbers that avoids the two reversees by using substr, pos and look-around assertions in the regex.

knoppix@Microknoppix:~$ perl -E '
> sub add_commas
> {
>     my $number = shift;
>     substr $number, pos $number, 0, q{,}
>        while $number =~ m{ (?<= \d ) (?= \d{3} (?: , | \z ) ) }xg;
>     return $number;
> }
> 
> say add_commas( $_ ) for qw{
>    1
>    12
>    123
>    1234
>    12345
>    123456
>    1234567
>    12345678
>    123456789
>    1234567890
>    };'
1
12
123
1,234
12,345
123,456
1,234,567
12,345,678
123,456,789
1,234,567,890
knoppix@Microknoppix:~$
[download]

I hope this is of interest.

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