$ perl -E '
> say join q{,}, map { sprintf q{%02d}, $_ } split m{,} for qw{
>    1,2,3,4
>    8,9,10,11
>    7,8,9,10
>    12,13,14,15
>    };'
01,02,03,04
08,09,10,11
07,08,09,10
12,13,14,15