You're going to have a lot of people doing a lot of different versions for this.. and here's mine!

#!/usr/bin/perl
use strict;
use warnings;

print (numformat(1000000),"\n");
print (numformat(123),"\n");
print (numformat(12),"\n");

sub numformat {
  my ($num) = @_;
  die "'$num' is not a simple integer" unless $num =~ /^\d+$/;
  while ($num =~ s/(\d)(\d\d\d)(?!\d)/$1,$2/) {};
  return $num;
}
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