I don't know a way to do so with sprintf...

Maybe the following might help you (about from perl-cookbook):

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

Best regards,
perl -le "s==*F=e=>y~\*martinF~stronat~=>s~[^\w]~~g=>chop,print"