Comma-ize any number bigger than 999.

commas.pl

#!/usr/bin/env perl

foreach $arg (0..$#ARGV) {
  my $num = int($ARGV[$arg]);
  my $fmt = "", $cnt = 0;

  while($num > 0) {
    $fmt = ($num % 10) . $fmt;

    if(++$cnt % 3 == 0 and $num >= 10) {
      $fmt = "," . $fmt;
    }

    use integer;
    $num /= 10;
  }

  print "$fmt\n";
}