sub process {
   my ($data, $filter) = @_;
   # lengthy calculations with $data here ...
   # in that length calculation you have to decide
   # if you continue with your calculation:

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

       }
   } else {
       push @$data, other_lengthy_calculation($_);
   }
}
# second option:
sub process {
   my ($data, $filter) = @_;

   # provide a default value for $filter
   $filter = sub {1;} unless $filter;

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

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