sub input    { my (        $c) = @_; ...; &$c($value ); }
sub process1 { my ($input, $c) = @_; ...; &$c($result); }
sub process2 { my ($input, $c) = @_; ...; &$c($result); }
sub output   { my ($result   ) = @_; ...;               }

input(
   sub { process1(@_, 
      sub { process2(@_,
         \&output
      ) }
   ) }
);