# third option:
sub process {
 my ($data, $filter) = @_;

 # lengthy calculations with $data here ... 
 # in that length calculation you have to decide
 # if you continue with your calculation:

 if ($filter) {
   for (@$data){
     if ($filter->($_)) {
       push @$data, other_lengthy_calculation($_);
     }
   }
 }
 else {
   for (@$data){
     push @$data, other_lengthy_calculation($_);
   }
 }