In your snippet of code, $bio_seq is a Bio::SeqIO object pointing to an output FASTA file

You can create such an object like so:

use strict;
use warnings;
use Bio::SeqIO;

my $bio_seq = Bio::SeqIO->new(
                  -file => '>outputfilename', 
                  -format => 'Fasta'
              );
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where: 'outputfilename' is the name of your desired output file. Friendly reminder: always remember to use the 'strict' pragma and enable 'warnings' to make sure you can identify problem areas in your code.