@f will hold an array of Bio::SeqFeatureI objects. This is one per feature in the feature table (so 'source', 'Protein', 'CDS' in this example). You have to further query the feature object to get the data out from it. A simple method $f->gff_string can be called to get a quick representation in GFF Format

To help you with your problem specifics, if you are trying to get the cds sequence for the protein, see the FAQ Q5.4 which should get you most of the way there.