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

output(
   process1(
      process2(
         input()
      )
   )
);