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