You could go through the protein's feature table and FIND the coded_by value. The trick is to associate the coded_by nucleotide coordinates to the nucleotide entry, which you'll retrieve USING the accession number from the same feature.

use Bio::Factory::FTLocationFactory;
use Bio::DB::GenPept;
use Bio::DB::GenBank;
my $gp = Bio::DB::GenPept->new;
my $GB = Bio::DB::GenBank->new;
# factory to turn strings into Bio::Location objects
my $loc_factory = Bio::Factory::FTLocationFactory->new;
 my $prot_obj = $gp->get_Seq_by_id($protein_gi);
foreach my $feat ( $prot_obj->top_SeqFeatures ) {
if ( $feat->primary_tag eq 'CDS' ) {
# example: 'coded_by="U05729.1:1..122"'
my @coded_by = $feat->each_tag_value('coded_by');
my ($nuc_acc,$loc_str) = split /:/, $coded_by[0];
my $nuc_obj = $gb->get_Seq_by_acc($nuc_acc);
# create Bio::Location OBJECT from a string
my $loc_object = $loc_factory->from_string($loc_str);
# create a Feature object by using a Location
my $feat_obj = Bio::SeqFeature::Generic->new(-location =>$loc_object);
# associate the Feature object with the nucleotide SEQ object
$nuc_obj->add_SeqFeature($feat_obj);
my $cds_obj = $feat_obj->spliced_seq;
print "CDS sequence is ",$cds_obj->seq,"n";
}
}

You could go through the protein's feature table and find the coded_by value. The trick is to associate the coded_by nucleotide coordinates to the nucleotide entry, which you'll retrieve using the accession number from the same feature.

use Bio::Factory::FTLocationFactory;
use Bio::DB::GenPept;
use Bio::DB::GenBank;
my $gp = Bio::DB::GenPept->new;
my $gb = Bio::DB::GenBank->new;
# factory to turn strings into Bio::Location objects
my $loc_factory = Bio::Factory::FTLocationFactory->new;
 my $prot_obj = $gp->get_Seq_by_id($protein_gi);
foreach my $feat ( $prot_obj->top_SeqFeatures ) {
if ( $feat->primary_tag eq 'CDS' ) {
# example: 'coded_by="U05729.1:1..122"'
my @coded_by = $feat->each_tag_value('coded_by');
my ($nuc_acc,$loc_str) = split /:/, $coded_by[0];
my $nuc_obj = $gb->get_Seq_by_acc($nuc_acc);
# create Bio::Location object from a string
my $loc_object = $loc_factory->from_string($loc_str);
# create a Feature object by using a Location
my $feat_obj = Bio::SeqFeature::Generic->new(-location =>$loc_object);
# associate the Feature object with the nucleotide Seq object
$nuc_obj->add_SeqFeature($feat_obj);
my $cds_obj = $feat_obj->spliced_seq;
print "CDS sequence is ",$cds_obj->seq,"n";
}
}