use List::Util qw(max);
my($self, $id) = @_;
my $dir = $self->{update_dir};
opendir(my $dir_h, $dir ) or return;

#Change this if you expect a possible valid id of "0"
my $id = max( grep { /^\d+$/ && $_ <= $id } readdir $dir_h )
  or return;
return "$dir/$id";
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