too tired to answer directly to the code, but for the last two questions see this code:

use strict;
use warnings;

my @numbers = 4 .. 6;
my $number_of_trailing_zeros = 3;
my $commify_at = 3;

@numbers = map {
    $_ .= "0" x $number_of_trailing_zeros;
    $_ =~ s/(\d{$commify_at})$/,$1/;
    $_;
} @numbers;

print "@numbers";
[download]

btw is there a special reason why some write use warnings before use strict?

anyway, HTH ;)