As a default value.

Suppose you write a sub (or module) that processes some values, and you want to offer the caller the possibility to filter some of them. Now you've got two options: 

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($_);
   }
}

[download]
You can see that the construct with the default value is much shorter (3 instead of ~8 lines), and it's much cleaner.

If you're interested in more details on callbacks, anonymous subs and many more advanced techniques, I strongly recommend the book "Higher Order Perl" by Mark Jason Dominus.

In reply to Re^3: what is sub {1;} by moritz
in thread what is sub {1;} by mlgvalt

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