what does data in $struct->{data} stands for?

$struct is a hash reference. data is an autovivified key in the referenced hash. (Update: The  -> arrow operator is needed because  $struct is a reference.) Each new hash reference built from each line of input read from an input file or from the  __DATA__ handle (as in the example code) is push-ed to the value of this key.

Also Is it possible to use without pretty?

Did you try it? What happened? (Update: And what does the documentation say?)