The Perl Cookbook (Recipe 2.17) has just what you need:

my $number = 987654321;
print commify($number);

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text
}
[download]

Update: blakem is right. And besides, my answer is incomplete. For a more complete answer, look here.

---

Sarah
If Bill Gates can name a company after his "bedroom" problems, I can have a stupid sig that points it out.