For learning Perl in general you may look at books mentioned here or tutorial. Bioinformatics related books are old but can be useful as this one has a chapter on parsing Genbank data. In addition to this you may look at BioPerl documentation.

For your case I created small script where 16S rRNA sequences are extracted from a bacteria with accession 'NC_013961'. For it to work you will need to install module Bio::DB::GenBank with cpan.

use strict;
use warnings;
use Bio::DB::GenBank;

my $gb = Bio::DB::GenBank->new();

# get data by accession
my $seqio = $gb->get_Stream_by_acc('NC_013961');
my $seq =  $seqio->next_seq;

foreach my $feat ( $seq->get_SeqFeatures() ) {
    # skip tags that are not "rRNA"
    next unless (($feat->primary_tag eq "rRNA") and $feat->has_tag("product"));

    # print sequence only if rRNA product tag value includes '16S'
    my @rRNA = $feat->get_tag_values('product');
    if ( grep( /^16S/, @rRNA ) ) {
      print $feat->seq->seq, "\n";
    }
}