If your question is how to convert a sequence file in fasta format to a sequence file in genbank format, then I'd suggest looking at Bio::SeqIO from BioPerl. That module makes formatting changes trivial:

use Bio::SeqIO;

my $in_obj  = Bio::SeqIO->newFh( '-file'   => $infile_name,
                                 '-format' => $infile_format );
my $out_obj = Bio::SeqIO->newFh( '-file'   => '>' . $outfile_name,
                                 '-format' => $outfile_format );

while( <$in_obj> )
{
    print $out_obj $_;
}
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If you want a custom format based on the genbank format, you could tweak the Bio::SeqIO::GenBank module. The relevant section of code (based on your original description) is in the write_seq function:

    # print out the sequence
    my $nuc = 60;        # Number of nucleotides per line
    my $whole_pat = 'a10' x 6; # Pattern for unpacking a whole line
    my $out_pat   = 'A11' x 6; # Pattern for packing a line
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