From the 'Perl Cookbook' (pages 64-65):

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

Is this more efficient? Heck, I don't know, but since no one had mentioned it...

Pat