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";